Konica Minolta bizhub C368: Paper Tray Has Paper but the Empty Paper Indicator Stays On

Introduction

A common paper-feed problem on the Konica Minolta bizhub C368 occurs when a paper tray contains paper but the machine still displays an empty-paper warning. In some cases, the tray lifting mechanism moves upward only a short distance and then stops. The paper-empty indicator may remain illuminated even though there is sufficient paper inside the tray.

This type of problem can be caused by more than a defective paper sensor. The relationship between the paper-quantity sensors, the tray upper-limit sensor, and the lifting mechanism must be understood before replacing parts.

For Tray 1, three sensors are particularly important:

• PS11: Near-empty paper sensor
• PS24: Paper-empty sensor
• PS25: Paper tray upper-limit sensor

These sensors have separate detection functions. PS11 and PS24 monitor paper quantity, while PS25 detects the position of the pickup pressure plate mechanism and is used by the main board to control the tray lifting motor M12.

A key diagnostic distinction is whether PS25 turns ON too early or remains OFF because the lifting mechanism cannot move correctly.

This guide explains the sensor logic, normal operating sequence, common failure conditions, and practical IO monitoring procedures for diagnosing paper tray problems on the bizhub C368.

Key Takeaways

• PS24 is the primary sensor associated with the paper-empty condition in Tray 1.
• PS11 provides a near-empty warning when the remaining paper quantity becomes low.
• PS25 detects the upper-limit position of the pickup pressure plate mechanism and controls the lifting process through the main board.
• PS25 turning ON too early may cause the lifting motor M12 to stop before the paper reaches the correct height.
• If PS25 remains OFF while the motor runs but the tray barely rises, inspect the lifting mechanism for mechanical resistance or transmission slippage.
• Tray 2 uses different sensor numbers: PS12, PS21, and PS22.
• IO monitoring combined with manual mechanical movement is an effective way to distinguish premature sensor activation from a mechanical lifting problem.

1. Understanding the PS11, PS24, and PS25 Sensors in Tray 1

The first step is to understand that the three sensors perform different functions.

According to the described C368 operating logic, PS11, PS24, and PS25 are independent photoelectric sensors. Each sensor sends its own ON/OFF status signal to the MFPB main board. They do not send commands directly to one another.

1.1 PS11 – Near-Empty Paper Sensor

PS11 detects when the remaining paper quantity reaches the near-empty warning level.

Its operation is driven by a paper-quantity lever that responds to the paper stack height.

When the remaining paper becomes low, the lever changes position and activates PS11. The machine can then display a near-empty warning.

PS11 does not control the tray lifting motor M12.

1.2 PS24 – Paper-Empty Sensor

PS24 detects the paper-empty condition in Tray 1.

It also responds to the position of a paper-quantity lever.

When the paper stack is exhausted, the lever changes position and activates PS24. The MFPB main board then recognizes the tray as empty and displays the corresponding paper-empty condition.

If the tray contains paper but PS24 remains in the empty-detection state, inspect the paper lever, sensor, and related mechanical movement.

1.3 PS25 – Paper Tray Upper-Limit Sensor

PS25 performs a different function.

It detects the position of a flag connected to the pickup pressure plate mechanism. The flag enters or leaves the sensor’s optical slot as the pressure plate moves.

The MFPB main board uses the PS25 signal to control the tray lifting motor M12.

In other words:

• PS11 monitors the near-empty paper condition.
• PS24 monitors the paper-empty condition.
• PS25 monitors the pickup pressure plate mechanism’s upper-limit position.

PS25 is not a paper-quantity sensor and does not directly control the paper-empty indicator.

2. Normal Paper Tray Lifting Mechanism

Understanding the normal lifting sequence makes it easier to identify premature sensor activation.

2.1 Normal Mechanical Relationship

The normal mechanical sequence is:

1. The tray lifting mechanism raises the paper platform.
2. The paper stack contacts the pickup pressure plate.
3. The pickup pressure plate rotates as it is pushed upward.
4. The connected flag moves into the PS25 optical slot.
5. PS25 changes to ON.
6. The MFPB main board stops the lifting motor M12.

The important point is that PS25 should respond to the actual movement of the pickup pressure plate mechanism.

2.2 Normal Operating Sequence When Paper Is Sufficient

During printing, paper is gradually consumed and the paper stack becomes thinner.

The normal lifting sequence is as follows:

1. As the paper stack becomes thinner, the pickup pressure plate moves slightly downward.
2. Its connected flag moves away from the PS25 optical slot.
3. PS25 changes to OFF.
4. The MFPB main board receives the OFF signal and starts M12.
5. The tray platform rises.
6. The paper stack pushes the pickup pressure plate upward again.
7. The flag enters the PS25 optical slot.
8. PS25 changes to ON, and M12 stops.

Under the described normal operating conditions, when the paper quantity is sufficient and the tray is in its stable upper position, the expected sensor states are:

PS25 being OFF is normally a temporary condition during the lifting cycle, rather than the expected stable state when the paper stack is sufficient.

3. Fault Condition A: M12 Moves Slightly and Stops, but PS25 Shows ON

This is an important diagnostic condition.

The tray platform rises only a short distance, the motor stops, and IO monitoring shows:

PS25 = ON

However, the paper stack has not yet pushed the pickup pressure plate upward.

Why does PS25 turn ON before the paper reaches the pressure plate?

Under normal mechanical conditions, this should not happen. It suggests that the flag, pressure plate, or sensor mounting position may have changed.

3.1 Deformed or Misaligned PS25 Flag

The plastic flag connected to the pickup pressure plate may become bent, deformed, or displaced.

Even though the pressure plate itself has not been pushed upward by the paper stack, the deformed flag may enter the PS25 optical slot after only a small amount of tray movement.

PS25 then changes to ON prematurely.

The MFPB main board interprets this signal as the upper-limit position having been reached and stops M12.

Inspection points:

• Check whether the plastic flag is bent or deformed.
• Check whether the flag has shifted from its original position.
• Observe whether the flag enters the PS25 optical slot too early.
• Compare the flag movement with the actual movement of the pressure plate.

3.2 Weak or Disconnected Pickup Pressure Plate Return Spring

The pickup pressure plate return spring may become weak, detached, or unable to return the plate to its correct position.

The pressure plate may remain partially raised instead of returning normally.

As a result, the connected flag may already be close to the PS25 optical slot. A very small upward movement of the tray platform can then trigger PS25.

Inspection points:

• Check whether the return spring is installed correctly.
• Check for spring fatigue or deformation.
• Check whether the pressure plate moves freely.
• Confirm that the pressure plate returns to its normal resting position.

3.3 Loose or Cracked PS25 Sensor Bracket

The PS25 sensor assembly may move because its mounting bracket is loose, cracked, or deformed.

Even if the mechanical flag moves normally, a displaced sensor can detect it at the wrong position.

This can cause PS25 to turn ON before the paper stack reaches the pickup pressure plate.

Inspection points:

• Check the PS25 mounting bracket.
• Look for cracks or deformation.
• Check whether the sensor assembly is loose.
• Confirm that the flag and optical slot are correctly aligned.

3.4 Typical Symptoms of Premature PS25 Activation

The following combination strongly suggests premature PS25 activation:

• M12 raises the tray platform only a short distance.
• M12 stops shortly afterward.
• IO monitoring shows PS25 = ON.
• The paper stack has not yet contacted the pickup pressure plate.
• There is still a visible gap between the paper and the pressure plate.

In this situation, inspect the flag, return spring, and sensor bracket before replacing the lifting motor.

4. Fault Condition B: M12 Moves Slightly, but PS25 Remains OFF

This condition should not be confused with premature PS25 activation.

If M12 continues running or attempts to lift the platform, but the tray barely moves and PS25 remains OFF, the problem is more likely related to mechanical resistance or transmission slippage.

Possible causes include:

4.1 Paper Tray Guide Resistance

Paper dust, toner contamination, or foreign objects may accumulate around the left and right tray guides.

Excessive friction can prevent the paper platform from moving upward smoothly.

Check:

• Left and right lifting guide areas
• Paper dust and toner accumulation
• Foreign objects
• Signs of mechanical deformation or abnormal friction

4.2 Lifting Wire Problems

The lifting wire may become frayed, loose, disconnected, or displaced from its pulley groove.

If the wire cannot transmit the lifting force correctly, the motor may operate without raising the platform sufficiently.

Check the wire condition and pulley engagement carefully.

4.3 Worn M12 Motor Gears or Slipping Lifting Clutch

Worn motor gears or a slipping lifting clutch can prevent the tray platform from reaching the required height.

Possible symptoms include:

• The motor can be heard running.
• The tray platform moves very little.
• PS25 remains OFF.
• The lifting process may eventually generate a lifting-related service error.

The specific error code should be confirmed against the machine’s actual service information.

5. PS25 Troubleshooting: Compare the IO Status with Mechanical Movement

The following table summarizes the key diagnostic differences.

Do not replace PS25 immediately just because the lifting process stops unexpectedly. First determine whether the sensor is receiving an incorrect mechanical signal or whether the sensor itself has failed.

6. Practical IO Monitoring and Mechanical Verification

IO monitoring is one of the most useful methods for distinguishing a sensor-position problem from a mechanical lifting problem.

Step 1: Enter Service Mode and Open IO Monitoring

Access the machine’s service mode and locate the relevant IO monitoring screen.

Monitor PS25 in real time.

Use the correct service procedure for the specific machine configuration.

Step 2: Slowly Raise the Tray Platform Manually

With the machine made safe for the inspection, slowly raise the tray lifting platform by hand.

Observe two events:

• The exact moment PS25 changes to ON.
• The moment the paper stack contacts the pickup pressure plate.

Keep hands clear of moving gears, cables, and other pinch points. Follow the appropriate service safety procedures.

Step 3: Compare the Two Events

Case A: PS25 turns ON before the paper contacts the pressure plate

This indicates premature sensor activation.

Inspect:

• The flag connected to the pressure plate
• The pressure plate return spring
• The PS25 mounting bracket

Case B: PS25 turns ON at approximately the same time the paper contacts the pressure plate

The sensor and mechanical relationship appear consistent with the described normal operation.

If the original fault remains, continue checking the lifting drive and other related components.

Case C: The platform reaches the upper position, but PS25 remains OFF

Check:

• PS25 optical window for contamination
• Sensor operation
• Wiring and connectors
• Sensor and flag alignment

A failed sensor or wiring problem may prevent the main board from receiving the expected ON signal.

7. How PS11, PS24, and PS25 Relate to the Paper-Empty Indicator

A critical distinction in this troubleshooting process is that the paper-empty indicator and the tray lifting motor do not rely on the same sensor.

According to the described C368 sensor logic:

• PS24 controls the paper-empty detection for Tray 1.
• PS11 provides the near-empty warning.
• PS25 controls the upper-limit detection used in the M12 lifting process.

PS25 does not directly trigger the paper-empty indicator.

7.1 Sensor Function Comparison

7.2 Paper Quantity Detection Sequence

As paper is consumed, the sensor states change according to the remaining paper quantity.

The exact display behavior should be checked against the relevant machine configuration and panel indication.

7.3 Paper Is Present but the Empty Indicator Stays On

If Tray 1 contains paper but the empty-paper indicator remains illuminated, check the following in order:

1. Confirm that the tray is fully inserted.
2. Inspect the PS24 paper-quantity lever for sticking or abnormal movement.
3. Check whether the PS24 optical sensor is contaminated.
4. Monitor the PS24 IO status.
5. Check the sensor wiring and connector if the mechanical lever operates normally but the IO state is incorrect.

Do not assume that a PS25 fault is responsible for the empty-paper indicator. PS25 is primarily related to the lifting process in the described system.

7.4 Near-Empty Warning Versus Paper-Empty Indication

These two conditions should also be distinguished.

• PS11 = ON and PS24 = OFF: the machine detects a near-empty paper condition.
• PS24 = ON: the machine detects an empty-paper condition.

A near-empty warning does not necessarily mean that the tray cannot continue printing.

8. Paper-Empty Indicator Flashing vs. Staying On

The indicator behavior can provide another useful diagnostic clue.

According to the described C368 logic:

If the Indicator Flashes but Paper Is Still Present

Prioritize checking:

• PS11 paper-quantity lever
• PS11 optical window
• PS11 sensor operation
• Sensor wiring if the mechanical movement is normal

If the Indicator Stays On but Paper Is Still Present

Prioritize checking:

• PS24 paper-quantity lever
• PS24 optical window
• PS24 sensor operation
• Paper tray installation and seating
• Related wiring and connectors

If M12 Has a Lifting Problem but the Paper Indicator Works Normally

Focus on PS25 and the lifting mechanism.

A lifting problem and a paper-empty detection problem should be diagnosed separately unless IO monitoring shows that both conditions are present.

9. Tray 2: Paper Present but Empty Indicator Stays On and the Platform Only Rises Slightly

Tray 2 uses different sensor numbers from Tray 1.

The operating principle is similar, but the sensor identification must not be mixed up.

9.1 Tray 1 vs. Tray 2 Sensor Number Comparison

These are separate hardware and detection circuits.

When troubleshooting Tray 2, use PS12, PS21, and PS22 rather than the Tray 1 sensor numbers.

9.2 Typical Tray 2 Fault Symptoms

A typical reported condition is:

• Tray 2 contains paper.
• The tray is fully inserted.
• The lifting motor operates.
• The paper platform rises only a short distance and then stops.
• The control panel displays a paper-empty warning.
• The Tray 2 empty-paper indicator remains illuminated.

In this situation, premature PS22 activation may be responsible.

9.3 Possible Fault Sequence: PS22 Activates Too Early

The described fault sequence is:

1. PS22 changes to ON prematurely.
2. The MFPB main board stops the Tray 2 lifting motor M13.
3. The paper platform stops before reaching the correct height.
4. The paper stack does not push the pickup pressure plate into its normal position.
5. The PS21 paper-empty lever remains in the empty-detection position.
6. PS21 stays ON.
7. The machine detects Tray 2 as empty and displays the empty-paper warning.

This explains why a tray can contain paper but still be detected as empty: the lifting platform has not raised the paper stack far enough to operate the paper-quantity detection mechanism correctly.

9.4 Common Causes of Premature PS22 Activation

The possible causes are similar to those of premature PS25 activation in Tray 1.

Deformed pickup pressure plate flag

A bent or displaced flag may enter the PS22 optical slot after only a small amount of platform movement.

Weak or disconnected return spring

The pickup pressure plate may remain partially raised, allowing the flag to activate PS22 too early.

Loose or damaged PS22 bracket

A loose or cracked sensor bracket can change the relative position between the flag and the optical sensor.

Inspect the mechanical relationship before replacing the sensor.

9.5 Tray 2 IO Monitoring Procedure

The following procedure helps confirm whether PS22 is activating too early.

Step 1: Open IO Monitoring

Enter the appropriate service mode and monitor PS22 and PS21.

Step 2: Check the Initial Sensor States

With Tray 2 fully inserted and sufficient paper loaded, check the initial sensor states.

According to the described operating sequence:

• PS22 should initially be OFF before the lifting process.
• PS21 should indicate that paper is available.

Step 3: Observe the Lifting Process

Start the lifting process and monitor both sensor signals.

If PS22 changes to ON after only a small amount of movement, causing M13 to stop while PS21 remains ON, premature PS22 activation is strongly indicated.

Step 4: Manually Raise the Platform

Carefully raise the platform manually until the paper stack pushes the pickup pressure plate.

Observe whether PS21 changes to OFF and the paper-empty warning disappears.

If this occurs, it supports the diagnosis that the platform was stopping too early and preventing normal paper-quantity detection.

9.6 How to Distinguish Premature PS22 Activation from Transmission Slippage

A mechanical transmission problem can produce similar symptoms, but the IO status is different.

The key is to observe the PS22 signal during the actual lifting process rather than relying only on the panel warning.

10. Common Diagnostic Mistakes

Mistake 1: Replacing the Paper-Empty Sensor Immediately

A paper-empty warning does not automatically mean PS24 or PS21 is defective.

The lifting platform may have stopped too early because the upper-limit sensor activated prematurely.

Check the upper-limit sensor and lifting sequence before replacing the paper-quantity sensor.

Mistake 2: Confusing PS25 with PS24

PS25 and PS24 perform different functions.

• PS25 detects the pickup pressure plate mechanism’s upper-limit position.
• PS24 detects the paper-empty condition.

Replacing PS25 will not necessarily solve a genuine PS24 detection problem.

Mistake 3: Treating Every Short Lifting Movement as a Motor Failure

If the motor moves only a short distance and stops, do not immediately replace M12 or M13.

First check whether the corresponding upper-limit sensor has changed to ON prematurely.

Mistake 4: Ignoring the Mechanical Flag and Sensor Bracket

An optical sensor can operate normally but still produce an incorrect detection result if its mechanical flag or mounting position has changed.

Always inspect the complete mechanical relationship.

Mistake 5: Using Tray 1 Sensor Numbers to Diagnose Tray 2

Remember the sensor number differences:

• Tray 1: PS11, PS24, PS25
• Tray 2: PS12, PS21, PS22

Using the wrong sensor identification can lead to incorrect troubleshooting conclusions.

11. Quick Troubleshooting Checklist

Use this checklist when a Konica Minolta C368 tray contains paper but displays an empty-paper warning.

Tray 1

Tray 2

12. Frequently Asked Questions

Q1: Why does my Konica Minolta C368 show a paper-empty warning when the tray still contains paper?

Possible causes include a stuck paper-quantity lever, a defective or contaminated PS24 sensor, a wiring problem, or a lifting mechanism that stops before the paper reaches the correct detection position.

For Tray 1, monitor PS24 and PS25 to determine whether the problem is related to paper detection or premature lifting termination.

Q2: What is the difference between PS24 and PS25?

PS24 detects the paper-empty condition. PS25 detects the upper-limit position of the pickup pressure plate mechanism and is used to control the lifting motor M12.

They have different functions and should not be treated as interchangeable sensors.

Q3: Can a faulty PS25 sensor cause the paper-empty indicator to stay on?

PS25 does not directly control the paper-empty indicator in the described C368 logic.

However, if PS25 activates too early, the lifting motor may stop before the paper stack reaches the pickup pressure plate. This can indirectly cause the paper-quantity mechanism to remain in an empty-detection position.

Q4: Why does the tray platform rise slightly and then stop?

One possible cause is premature activation of the upper-limit sensor.

For Tray 1, check PS25. For Tray 2, check PS22.

If the sensor remains OFF while the motor operates but the platform cannot rise, inspect the mechanical lifting system for resistance or transmission slippage.

Q5: What should I check if PS25 stays OFF?

First determine whether the tray platform can reach the correct upper position.

If it cannot, inspect the lifting mechanism, including the tray guides, lifting wire, clutch, and motor gears.

If the platform reaches the expected upper position but PS25 remains OFF, check the sensor, optical window, wiring, and flag alignment.

Q6: What are the sensor numbers for Tray 2?

For the described bizhub C368 configuration:

• PS12: Near-empty paper sensor
• PS21: Paper-empty sensor
• PS22: Upper-limit sensor

The lifting motor is M13.

Q7: What is the difference between a flashing and a continuously illuminated paper indicator?

According to the described sensor logic, a flashing indication is associated with the near-empty condition detected by PS11, while a continuously illuminated empty-paper indication is associated with PS24.

Confirm the exact panel behavior for the machine configuration being serviced.

Conclusion

When a Konica Minolta bizhub C368 paper tray contains paper but the machine displays an empty-paper warning, the problem should not be diagnosed by replacing the paper sensor alone.

The most important step is to distinguish between paper-quantity detection and the tray lifting control system.

For Tray 1, PS11 monitors the near-empty condition, PS24 detects the paper-empty condition, and PS25 controls the upper-limit detection used in the M12 lifting process.

For Tray 2, the corresponding sensors are PS12, PS21, and PS22, with M13 controlling the lifting process.

If the lifting motor stops after a short movement and the upper-limit sensor changes to ON before the paper contacts the pickup pressure plate, inspect the flag, return spring, and sensor bracket for premature activation.

If the upper-limit sensor remains OFF while the platform cannot rise, focus on mechanical resistance and transmission components.

Monitoring the relevant IO signals while observing the actual mechanical movement is the most effective way to separate these two fault conditions.

Technical note: This article describes the sensor logic and repair observations supplied for the Konica Minolta bizhub C368. Confirm sensor states, service-mode procedures, error-code definitions, and component configurations against the applicable service documentation before performing repairs. The diagnostic principles should not be assumed to apply identically to every Konica Minolta model.