Error Code C3722 is a fusing abnormally high temperature detection error on Konica Minolta bizhub machines. It fires when a thermistor inside the fusing unit reads a temperature that exceeds a predetermined safety threshold for a continuous period — signalling that the fusing unit is overheating, or that a thermistor has failed and is reporting a false high-temperature reading.

This error is a hard protection event — the machine halts immediately to prevent a fire hazard, belt or roller damage, and potential injury. It will not self-clear and requires both identification of the root cause and a technician-level reset before printing can resume.

Critical: C3722 Has Different Meanings by Model Family

Unlike many bizhub error codes that carry the same meaning across all models, C3722 is mapped to different thermistors and detection conditions depending on which machine family you are working on. Diagnosing the wrong sensor wastes time and leads to unnecessary parts replacement. Before starting any troubleshooting, confirm your model and use the correct reference below.

What Does Error Code C3722 Mean?

Inside every bizhub fusing unit, multiple thermistors continuously measure the temperature of the heating roller (and belt on belt-fusing models) at different positions along its length:

• TH1 — typically measures the center of the heating roller surface
• TH2 — typically measures one edge of the heating roller (the trailing or leading edge depending on model)
• TH3 — on high-volume models (654/754 series), measures the opposite edge
• TEMS — a non-contact temperature sensor on some models, used as a secondary reference

These sensors feed continuous temperature data to the control board (PRCB or MFPB), which uses it to regulate the fusing heater lamps or IH coil. The machine maintains the fuser at a precise operating temperature — typically between 160°C and 200°C depending on paper type — by cycling the heater on and off based on these readings.

C3722 fires when the relevant thermistor (TH1, TH2, or TH3 depending on model) reports a temperature above the maximum safe threshold for a continuous predetermined period. Additionally, on the bizhub 654/754 series, it fires when the hard protection signal — a hardware safety interlock independent of the software temperature control — remains in the OFF state for longer than allowed.

The overtemperature reading can be genuine (the fuser is actually overheating) or false (the thermistor or its connector is producing an incorrect reading that the machine interprets as overheating). Both produce the same error code, but they require completely different remedies.

Common Causes of Error Code C3722

• Fusing unit not fully seated or incorrectly installed — the most common cause across all models. A fusing unit that is not completely pushed into position will have marginal connector contact. The thermistor signals become intermittent or noisy, causing the control board to read fluctuating or falsely high temperature values that trigger C3722
• Loose or oxidized fusing unit connector — the multi-pin connector between the fusing unit and the main harness can develop poor contact due to oxidation, thermal cycling, or physical displacement after service. An open-circuit or high-resistance thermistor connection causes the control board to read an incorrect (often maximum-scale) temperature value
• Right door interlock issue — if the right-side door does not fully close and engage its interlock switch, some models disable or alter fusing temperature control in a way that causes thermal runaway and genuine overtemperature, triggering C3722
• Failed or drifted thermistor (TH1 / TH2 / TH3) — the thermistor inside the fusing unit has degraded and now reads consistently above the actual fuser temperature. Thermistor failures typically produce a steady falsely-high reading rather than the fluctuating pattern associated with connector faults
• Faulty IHPU (IH Power Unit) — on IH-fusing models (C558/C658, 558e/658e, 654/754), a failing IHPU can cause uncontrolled heater output that genuinely overheats the fusing roller. The IHPU regulates the induction heating coil current — a failed regulation circuit removes the control loop and the temperature rises unchecked until C3722 fires
• Faulty FUPU (Fusing Power Unit) — on C458 and 458e models, the FUPU performs the same role as the IHPU on larger models. A failed FUPU triac or relay can cause heater overcurrent and genuine fuser overtemperature
• Faulty DCPU (DC Power Unit) — on models using a DCPU in the fusing circuit, a DCPU fault can affect fusing temperature regulation
• Noise filter (NF1) fault (654/754 series) — on the bizhub 654/754, the noise filter NF1 is in the AC path to the IHPU. A failed NF1 can cause the IHPU to receive distorted AC input, leading to uncontrolled heating
• Faulty PRCB or MFPB — in rare cases the control board’s thermistor input circuit or fusing heater control output has failed, causing either a false temperature reading or loss of heater regulation
• Paper jam in the fusing area — paper wrapped around the fusing roller or belt acts as insulation, trapping heat and causing localised overtemperature. C3722 appearing immediately after a paper jam clearance is a strong indicator of residual paper in the fusing unit

Step-by-Step Troubleshooting Procedure

Step 1 — Allow the Machine to Cool Fully Before Any Inspection

1. Power the machine OFF using the main power switch
2. Wait at least 20 minutes — longer if the machine was running a heavy job when C3722 occurred
3. Do not attempt to open the fusing area immediately — some models retain heat for 30 minutes or more after shutdown

Step 2 — Check the Right-Side Door and Interlock

Before touching the fusing unit itself, verify the right-side door closes and latches correctly. An improperly closing door is a frequently overlooked contributor to fusing temperature errors.

1. Open and close the right-side door several times — it should swing freely without resistance and close with a firm, positive click
2. With the door fully closed, press on it gently — it should not flex inward or feel loose
3. Inspect the door latch mechanism and the door interlock switch actuator for damage, wear, or misalignment
4. If the door does not close fully or the interlock switch is not being actuated correctly, correct this before proceeding — a door that appears closed but is not fully latched can cause fusing temperature regulation errors

Step 3 — Inspect the Fusing Unit for Correct Installation and Paper Fragments

1. Open the right-side door and examine the fusing unit installation:
   • Confirm the fusing unit locking lever is in the locked/secured position
   • Release the locking lever, pull the fusing unit out partially, then push it firmly back in until the connector engages and the unit stops — confirm the locking lever re-engages fully
2. While the fusing unit is accessible, use a flashlight to inspect the fusing unit entry and exit slots for paper fragments — even a small piece of paper inside the fusing unit can cause localised overtemperature by insulating the roller surface
3. Also inspect the roller surfaces for wrinkled paper or toner buildup that could cause uneven heat distribution
4. Remove any paper fragments with tweezers — never use sharp tools that could scratch the roller surface
5. Close the door, power on, and observe whether C3722 returns

Step 4 — Inspect and Reseat the Fusing Unit Harness Connector

Use the model-specific connector reference below to locate and reseat the correct connector for your machine.

bizhub C224 / C284 / C364 / C224e / C284e / C364e / C454 / C554 / C454e / C554e / C220 / C280 / C360 / C452 / C552 / C652 / C353 / C451:

1. Power OFF and unplug the machine, allow to cool
2. Access the fusing unit connector — trace the harness from the fusing unit to the main body
3. Reseat the connector at the fusing unit end and at the DCPU / PRCB junction
4. Inspect all pins for oxidation, bending, or toner contamination — clean with a dry cotton swab

bizhub 227 / 287 / 367 / C227 / C287 / C367 / 128DN / 136DN:

1. Power OFF and unplug the machine
2. Reseat the connector at: fusing unit → relay CN19 → MFPB CN18E
3. Inspect all three connection points in this path — the relay connector in the middle of the harness is a common failure point on this model family

bizhub C258 / C308 / C368 / 308e / 368e:

1. Reseat: fusing unit → MFPB (direct harness path)
2. Inspect the connector at the MFPB end for oxidation or pin damage

bizhub C458 / 458e:

1. Reseat: fusing unit → MFPB CN12E
2. Also reseat the connector between the FUPU and MFPB — the FUPU is in the fusing heat control path on these models

bizhub C558 / C658 / 558e / 658e:

1. Reseat: fusing unit → MFPB CN22E
2. Also check: TEMS/1 → relay CN109 → MFPB CN15E and TEMS/2 → relay CN138 → MFPB CN15E
3. Reseat the connector between the IHPU and MFPB

bizhub 654 / 654e / 754 / 754e:

1. Reseat: fusing unit → IHPU and IHPU → PRCB
2. Also check the connector at NF1 (noise filter) — this is unique to the 654/754 series and is frequently overlooked

Step 5 — Monitor Fusing Temperature in Service Mode

Before replacing any parts, use service mode to observe the actual thermistor readings. This immediately confirms whether the temperature is genuinely high or whether a sensor is producing a false reading.

1. Enter Service Mode (refer to your model’s service manual for the access procedure)
2. Navigate to the fusing temperature monitor or sensor check section — this is typically found under Adjust → Fusing or I/O Check → Fusing
3. Observe the temperature readings for all available fusing sensors (TH1, TH2, TH3, TEMS) while the machine attempts to warm up
4. A sensor reading at maximum scale (e.g. 250°C+ or full ADC range) immediately after a cold startup indicates a failed sensor or open-circuit connector — not a genuine overtemperature condition. The fuser physically cannot be that hot at cold start
5. A sensor reading that is genuinely elevated above the other sensors during warmup (e.g. TH2 reading 50°C higher than TH1 at the same time) suggests localised overheating — a paper fragment, roller surface contamination, or a failing heater control circuit
6. A sensor reading that is fluctuating erratically across all channels simultaneously suggests a connector fault or a failing power supply board (DCPU/FUPU/IHPU)

Step 6 — Replace the Fusing Unit

If connector checks are confirmed good, no paper fragments are present, the door closes correctly, and the temperature monitor shows a genuine overtemperature or a stable falsely-high reading from a specific thermistor, replace the fusing unit.

1. Order the correct fusing unit for your model and voltage — fusing unit part numbers are voltage-specific and differ between model generations
2. Power OFF and allow the machine to cool fully before removing the old unit
3. Install the replacement fusing unit, ensure the connector seats fully, and engage the locking lever
4. After installation, navigate to Service Mode → Counter and reset the fusing unit counter to zero
5. Run a gradation adjustment and several test prints across different paper sizes — confirm the temperature readings in service mode are stable and within the normal operating range across all sensors

Step 7 — Replace the IHPU, FUPU, or DCPU (Model Dependent)

If a confirmed-good fusing unit still produces C3722 and the temperature monitor shows erratic or genuinely runaway temperature readings, the heater power supply board has a failed regulation circuit.

• Replace IHPU — bizhub C558/C658, 558e/658e, 654/654e/754/754e (Part numbers: C658/C558 = A8KNM40A02)
• Replace FUPU — bizhub C458, 458e (Part number: C458 = A79MM40N00)
• Replace DCPU — bizhub C224/C284/C364 family, C454/C554 family, C220/C280/C360, C452/C552/C652

After replacing the heater power supply board, run a full warm-up cycle and monitor fusing temperatures in service mode to confirm stable regulation before returning the machine to service.

Step 8 — Replace the Noise Filter NF1 (bizhub 654 / 754 Series Only)

On the bizhub 654/654e/754/754e, the noise filter NF1 is in the AC supply path to the IHPU. A degraded NF1 passes distorted AC to the IHPU, affecting its ability to regulate the IH coil correctly. If IHPU replacement does not resolve C3722 on these models, replace NF1 before replacing the PRCB.

Step 9 — Replace the PRCB or MFPB

Board replacement is the last resort for C3722. Only consider it after the fusing unit, heater power supply board, and all connectors have been confirmed good.

• PRCB — bizhub C224/C284/C364 family, C454/C554, C220/C280/C360, C452/C552/C652, C353/C451/C550/C650, 654/754
• MFPB — bizhub C227/C287/C367, 227/287/367, C258/C308/C368, C458/C558/C658, 308e/368e/458e/558e/658e, 128DN/136DN

Quick Reference — Troubleshooting by Symptom

Understanding the C372x and C382x Fusing Temperature Error Family

C3722 belongs to a group of fusing temperature errors — both high and low — that cover every thermistor and temperature sensor in the fusing unit. Understanding where C3722 sits in this family helps diagnose related codes that may appear alongside it:

• C3721 — Fusing high temperature: Thermistor/1 (TH1) or heating side main thermistor
• C3722 — Fusing high temperature: Thermistor/2 (TH2) / edge thermistor / TH3 on 654/754
• C3723 — Fusing high temperature: Thermistor/3 (on models with three thermistors)
• C3725 — Fusing high temperature: TEMS (temperature sensor) / heating side
• C3726 — Fusing high temperature: TH2 (on some model families — check service manual for your model)
• C3731 — Fusing high temperature: Pressure side thermistor
• C3821 — Fusing low temperature: Thermistor/1 (TH1)
• C3822 — Fusing low temperature: Thermistor/2 (TH2)

C3721 and C3722 appearing together on the same machine typically points to a fusing unit connector fault that is affecting all thermistor signals simultaneously — a single connector reseat often clears both codes at once.

C3722 (high temp) followed shortly by C3822 (low temp on same sensor) is a classic pattern of a failing thermistor that is drifting out of calibration — readings oscillate above and below the thresholds. Replace the fusing unit in this case.

Preventing C3722 From Recurring

• Inspect the fusing unit connector at every PM visit — toner dust and thermal cycling gradually degrade contact quality. A brief reseat and inspection during routine maintenance prevents the majority of connector-related C3722 calls
• Replace the fusing unit on schedule — most bizhub fusing units are rated for 150,000–300,000 pages depending on model. Running beyond the rated life increases the risk of thermistor drift, heater lamp degradation, and roller surface wear — all of which contribute to C3722
• Always verify no paper remains in the fusing unit after a jam — paper fragments in the fusing area are one of the most preventable causes of C3722. Use a flashlight and spend 60 seconds confirming the path is clear after every fusing jam clearance
• Ensure the right-side door closes properly — a door that does not fully latch affects fusing temperature regulation on many bizhub models. Check the door mechanism during every service visit and replace worn door latches proactively
• Monitor IHPU/FUPU condition on high-volume machines — the heater power supply boards have a finite service life on IH-fusing models. On machines above 500,000 pages with a history of intermittent C3722 events, consider proactive IHPU/FUPU replacement as part of major servicing

Professional Technician Summary

Error Code C3722 on Konica Minolta bizhub machines is a fusing overtemperature detection error, and the most important diagnostic step is confirming whether the temperature reading is genuine or false before replacing any parts. Monitoring the thermistor readings in service mode immediately after a cold startup takes two minutes and tells you everything you need to know — a maximum-scale reading at cold start is a connector fault, not a real overtemperature event.

In field practice, a fusing unit that is not fully seated is the leading cause of C3722 across all model families. The second most common cause is a loose connector in the thermistor harness path. Genuine thermistor failures and power supply board faults are less common but should be the next suspects once installation and connectors have been confirmed.

On IH-fusing models (C558/C658, 558e/658e, 654/754), always check the IHPU before replacing the fusing unit when the temperature monitor shows genuinely elevated or runaway readings — a failing IHPU is a hardware fault that will cause the same overtemperature condition with any fusing unit installed, and replacing the fuser alone will not resolve it.