Error Code C5351 appears on a wide range of Konica Minolta bizhub machines and signals that the power supply cooling fan (FM1) has failed to turn — or more precisely, that the fan’s lock signal is reporting an abnormal state that does not match the control board’s command. When this error fires, the machine halts immediately to protect internal components from overheating damage.

Cooling fan errors are among the most straightforward to diagnose on the bizhub platform — the component chain is short, the failure modes are predictable, and in most cases the fix is a connector reseat or a fan replacement. This guide covers every affected model, explains exactly what the lock signal detection means, and walks through the complete technician-level repair procedure.

All Affected Models and Connector References

C5351 is documented across multiple bizhub generations. The fan designation, connector path, and control board differ between model families. Use the table below to identify your exact configuration before starting diagnosis.

What Does Error Code C5351 Mean?

Every cooling fan in the bizhub machine has a built-in lock signal — a feedback mechanism that tells the control board whether the fan is actually spinning. This signal is generated by a Hall-effect sensor or encoder inside the fan motor. When the fan rotates, the lock signal pulses continuously. When the fan is stopped, the signal stays in a fixed state.

The control board monitors this lock signal in real time and compares it against its commands. C5351 fires under the following conditions:

• Lock signal remains HIGH for longer than the allowed threshold while the fan is commanded to run — the control board is telling the fan to spin, but the lock signal is stuck, indicating the fan blades are not rotating
• Lock signal remains LOW for longer than the allowed threshold while the fan is stopped — on some models, this second condition is also monitored and can indicate a shorted fan motor or stuck signal line

The machine stops immediately when either condition is detected because the power supply cooling fan is not optional equipment — it removes heat from the high-voltage power supply board. Without it, power supply components overheat within minutes, causing permanent damage to the DC power supply unit, the AC board, and in severe cases the PRCB or MFPB.

Common Causes of Error Code C5351

Understanding the likely root causes before starting saves diagnostic time:

• Fan blades obstructed by debris — paper dust, toner particles, or a foreign object lodged in the fan housing prevents the blades from spinning freely. The motor receives power but cannot overcome the mechanical resistance, and the lock signal stays HIGH. This is the most common cause on older machines and those in dusty environments
• Fan motor seized — the fan motor’s bearings have worn out or corroded, causing the rotor to seize. A seized fan may still hum when power is applied but will not rotate. Common in older machines or those in humid environments
• Loose or disconnected connector — the fan receives no power or the lock signal line is broken at a connector, causing the board to read an abnormal signal state. Common after a PM visit, fan cleaning, or any service that required opening the machine
• Blown ICP2 on PRCB — the inline circuit protector on the PRCB that feeds FM1 has blown due to a transient overcurrent event. The fan connector may be perfect but FM1 receives no power
• Failed fan motor (internal) — the Hall-effect sensor inside the fan motor has failed, producing an incorrect lock signal even when the blades are spinning normally. Less common but possible on high-hour fans
• Faulty PRCB or MFPB — in rare cases the control board’s fan drive circuit has failed, preventing it from correctly powering or monitoring FM1

Step-by-Step Troubleshooting Procedure

Step 1 — Power Cycle the Machine

Before any physical inspection, perform a full power cycle. A transient voltage event during startup can occasionally cause the fan lock signal to misread, triggering C5351 without a real hardware fault.

1. Press the main power switch to OFF
2. Wait a full 30 seconds
3. Power the machine back on and listen carefully during the warm-up sequence — you should be able to hear the cooling fans spinning up
4. If C5351 clears and does not return on subsequent restarts, monitor the machine. A one-time occurrence is not necessarily a hardware fault
5. If C5351 returns consistently, proceed to Step 2

Step 2 — Locate FM1 and Inspect for Physical Obstructions

The power supply cooling fan (FM1) is typically located near the high-voltage power supply board, usually accessible from the right or rear side of the machine.

1. Power OFF and unplug the machine
2. Open the right-side or rear cover to access the fan — refer to your model’s service manual for the exact location, as FM1 position varies between model families
3. Visually inspect the fan blades — look for accumulated paper dust, toner buildup, a piece of torn paper, or any foreign object caught between the blades or in the fan housing
4. Using a can of compressed air, blow out any dust accumulation from the fan blades and housing — hold the blades stationary while blowing to prevent them from spinning as a generator and damaging the motor
5. After cleaning, manually spin the fan blades by hand — they should rotate freely and smoothly with almost no resistance. Any stiffness, grinding, or catching indicates bearing wear or debris still present
6. If the fan spins freely after cleaning, power on and test — accumulated dust creating drag is a common cause of intermittent C5351 on high-volume machines
7. If the fan is stiff or will not spin freely even after cleaning, the bearings are worn and FM1 must be replaced — proceed to Step 5

Step 3 — Inspect and Reseat the FM1 Connector

If the fan spins freely but C5351 persists, check the electrical connection. Use the connector reference for your model from the table above.

1. With the machine powered OFF and unplugged, trace the FM1 harness from the fan to its connector at the relay board and then to the PRCB or MFPB
2. Disconnect and firmly reseat the connector at FM1 → relay connector → PRCB CN18 (or the equivalent path for your model — refer to the table above)
3. Also reseat the connector at the relay board to PRCB/MFPB junction — this second connector is often overlooked and is a frequent source of intermittent C5351
4. Inspect every pin in both connectors for: bent or backed-out pins, green or white oxidation, cracked housing, or visible harness damage
5. If oxidation is present, clean the pins with electrical contact cleaner and a cotton swab — allow to dry fully before reconnecting
6. Power on and test — if C5351 clears after a connector reseat, monitor the machine. If the error returns, the connector housing is faulty and should be replaced

Step 4 — Perform FM1 Operation Check in Service Mode

With the fan confirmed mechanically free and connectors confirmed seated, use service mode to directly command FM1 and verify its response.

1. Enter Service Mode (refer to your model’s service manual for the entry procedure)
2. Navigate to the fan load check / I/O check section
3. Command FM1 to run using the fan operation check function:
   • bizhub C224/C284/C364 and e-series: Monitor at PRCB CN18-13 (REM) and PRCB CN18-15 (LOCK)
   • bizhub 458e/558e/658e: Check code 42, Multi code 1 — monitor at MFPB CN31E-9 (REM) / CN31E-11 (LOCK)
   • bizhub 368e/308e: Check code 42, Multi code 1 — monitor at MFPB CN24EA-9 (REM) / CN24EA-11 (LOCK)
   • bizhub 223/283/363/423: FM1 operation check via PRCB CN3
4. Observe the fan during the operation check — it should start immediately, run smoothly, and produce an audible airflow sound
5. If FM1 does not respond to the operation check command: confirm the drive signal is present at the connector using a multimeter. If the signal is present but the fan does not run, FM1 has failed — replace it (Step 5). If no signal is present, check ICP2 (Step 6) before suspecting the board
6. If FM1 runs during the operation check but C5351 still appears at startup: the fan is intermittently failing under thermal or load conditions — replace FM1 proactively

Step 5 — Replace Fan Motor FM1 (or M4 / M21)

If the fan is mechanically seized, fails the operation check, or runs intermittently, it must be replaced. Cooling fans have finite bearing life and are consumable components.

1. Order the correct replacement fan for your model — part numbers differ between model generations and fan designations (FM1, M4, M21). Always use a genuine Konica Minolta replacement or a high-quality OEM-equivalent with matching voltage and airflow specifications
2. Power OFF and unplug the machine
3. Disconnect the fan harness, remove the mounting screws (typically two or four), and extract the fan from its housing
4. Before installing the new fan, clean the fan housing and surrounding area of any accumulated dust — this prevents the replacement fan from being immediately burdened by the same dust load that may have contributed to the original failure
5. Install the replacement fan, ensuring the airflow direction arrow on the fan frame matches the original orientation — installing a fan backwards will reduce cooling efficiency significantly and can cause recurring overheating issues
6. Reconnect the harness and confirm the connector is fully seated
7. Power on and listen for the fan spinning during warm-up — confirm C5351 does not return
8. Run the FM1 operation check in service mode one more time to confirm the lock signal is responding correctly through the new fan

Step 6 — Check ICP2 Continuity (PRCB Models)

On models where the PRCB controls FM1, an Inline Circuit Protector (ICP2) sits in the fan power supply path on the board. This is a surface-mount fuse that protects the board’s fan drive circuit from overcurrent events caused by a seized or shorted fan motor. If ICP2 has blown, FM1 receives no power regardless of the fan’s condition.

1. With the machine powered OFF and unplugged, access the PRCB
2. Locate ICP2 on the board — refer to your model’s service manual for the exact component location on the PRCB (it is typically marked on the board’s silk screen as ICP2)
3. Set your multimeter to continuity mode and test ICP2 — a good ICP2 shows continuity (beep / near-zero resistance). An open circuit (no continuity) confirms ICP2 has blown
4. A blown ICP2 is a surface-mount component — board-level repair by a qualified technician is required to replace it, or the entire PRCB must be replaced
5. If ICP2 has blown, also replace FM1 before powering on — the original fan seizure is what caused ICP2 to blow in the first place. Running the machine with a blown ICP2 replaced but the same seized fan will blow the new ICP2 immediately

Step 7 — Replace the PRCB or MFPB

PRCB or MFPB replacement is the final step and should only be considered after FM1 has been confirmed good, all connectors have been reseated, and ICP2 continuity has been verified. A board fault that causes C5351 in isolation — where the fan, connectors, and ICP are all confirmed good — means the board’s fan drive transistor or lock signal input circuit has failed.

1. Replace the appropriate board for your model:
   • PRCB — for bizhub C224/C284/C364 series, C224e/C284e/C364e, 454e/554e, 223/283/363/423, 250/350, C250/C650, C353/C450, C7822/C7828
   • MFPB — for bizhub 458e/558e/658e, 368e/308e, C258/C308/C368, C458/C558/C658, 195/215/235
2. After board replacement, run the FM1 operation check in service mode to confirm the new board drives the fan correctly and reads the lock signal accurately
3. Power cycle the machine and confirm C5351 does not return during startup

Quick Reference — Troubleshooting by Symptom

Understanding the C535x Cooling Fan Error Family

C5351 belongs to a family of cooling fan error codes on the bizhub platform. Each code refers to a specific fan in the machine’s cooling system. If you are seeing multiple fan errors simultaneously, suspect a shared power supply issue or a control board fault rather than multiple simultaneous fan failures:

• C5351 — Power supply cooling fan (FM1 / M4 / M21) failure to turn
• C5352 — Secondary cooling fan (FM2) failure to turn
• C5353 — Imaging unit (IU) cooling fan failure to turn
• C5354 — Ozone ventilation fan failure to turn
• C5355 — Toner bottle cooling fan failure to turn
• C5356 — Main cooling fan (FM2) failure to turn — bizhub 227/287/367 series

If two or more of these codes appear together at startup, check the common power rails feeding multiple fans before replacing individual fan motors — a blown fuse or failed board power rail is a more efficient explanation than multiple simultaneous fan failures.

Preventing C5351 From Recurring

• Clean FM1 at every PM visit — the power supply cooling fan is typically positioned in an area of the machine that accumulates paper dust rapidly. A 2-minute compressed-air cleaning during routine maintenance significantly extends fan bearing life and prevents the dust-induced drag that causes premature seizure
• Check the FM1 connector during every service visit — the connector is in an area subject to vibration from the fan itself. A 30-second reseat during PM prevents the most common connector-related C5351 calls
• Replace FM1 proactively on high-volume machines — cooling fans are consumable mechanical components. On machines running above 10,000 pages per month, consider replacing FM1 as part of the standard PM kit rather than waiting for a failure, particularly on machines with a history of C5351 events
• Ensure the machine has adequate ventilation — bizhub machines require clearance on all sides for airflow. A machine pushed against a wall, enclosed in a cabinet without ventilation, or in a room with ambient temperatures above 30°C will cause FM1 to run continuously at maximum load, accelerating bearing wear significantly
• Use genuine replacement fans — third-party cooling fans often have lower bearing quality ratings, shorter MTBF specifications, and imprecise lock signal circuitry that can produce false C5351 readings even when the fan is physically spinning correctly

Professional Technician Summary

Error Code C5351 is one of the most straightforward error codes on the Konica Minolta bizhub platform to diagnose and resolve. The component chain is short — fan, connector, relay, board — and the failure modes are predictable.

In field practice, dust-obstructed or seized fan bearings account for the majority of C5351 cases, particularly on machines in office or industrial environments that accumulate paper dust quickly. Loose connectors after service visits are the second most common cause. ICP2 failures and board faults are uncommon and should only be considered after the fan and connectors have been definitively ruled out.

Always replace FM1 before checking ICP2 if the fan is confirmed seized — a blown ICP2 on an otherwise healthy board is a consequence of the seized fan, not the primary fault. Replacing ICP2 without also replacing the seized fan will blow the new ICP2 on the first startup.