Error Code C3425 is a fusing warm-up failure on Konica Minolta bizhub machines. It fires when the fusing unit cannot reach its required operating temperature within the allowed warm-up time — or when the temperature drops back below the minimum threshold after warm-up is declared complete. The machine halts immediately to prevent unfused toner from contaminating the paper path and to protect the fusing unit from damage caused by prolonged low-heat operation.

Unlike C3722 (which fires when the fuser overheats), C3425 is always a failure to heat. The fusing unit is not getting hot enough, not getting hot fast enough, or losing heat too quickly during operation. Understanding which of the three detection conditions triggered the error points you directly to the most likely root cause.

C3425 Has Different Sensor References by Model Family

Like most fusing error codes on the bizhub platform, C3425 maps to different sensors and power supply boards depending on the model family. Using the correct references for your machine before starting diagnosis prevents wasted time and unnecessary parts replacement.

What Does Error Code C3425 Mean?

The bizhub fusing unit must reach a precise operating temperature before printing can begin. On startup, the control board commands the heater power supply board (DCPU, FUPU, or IHPU depending on model) to energise the fusing heater lamps or IH coil. The TEMS (temperature sensor) inside the fusing unit monitors the heating roller temperature and reports it back to the control board in real time.

C3425 fires under any of three conditions — all of which represent a failure to heat correctly:

• Trigger 1 — Insufficient temperature rise rate during warm-up: The TEMS reading is not climbing fast enough from cold. The machine expects to see the temperature rising at a minimum rate during the first phase of warm-up. If the rate is too slow — because the heater lamp is weak, the heater power board is not delivering full power, or the TEMS connector is making poor contact — C3425 fires before the machine even reaches its target temperature
• Trigger 2 — Warm-up timeout: The machine starts its warm-up timer when heater power is commanded on. If the TEMS does not read the target warm-up temperature before this timer expires, C3425 fires. A completely failed heater lamp, a blown fuse in the heater circuit, or a disconnected fusing unit connector will all cause this condition
• Trigger 3 — Post warm-up temperature drop: The machine declares warm-up complete, but the TEMS then reads below the minimum operating threshold. This happens when the heater lamp is marginal — strong enough to pass warm-up but unable to sustain operating temperature under the thermal load of printing — or when the TEMS connector has an intermittent fault that causes sporadic low readings

Common Causes of Error Code C3425

• Fusing unit not fully seated — the most common cause of sudden-onset C3425 across all models. A fusing unit that is not completely pushed into its position will have marginal connector contact. The TEMS signal becomes intermittent and reads falsely low, or the heater lamp circuit is broken at the connector, preventing the fuser from heating entirely
• Loose or oxidized fusing unit harness connector — the multi-pin connector between the fusing unit and the machine harness can develop poor contact through oxidation or physical displacement after service. A high-resistance connection on the TEMS signal line causes the sensor to read lower than the actual temperature, triggering C3425 even when the fuser is heating normally
• Right-side door not fully closed — several bizhub models interrupt heater power or disable fusing temperature control when the right door interlock switch is not fully engaged. A door that appears closed but is not fully latched will prevent the fuser from reaching temperature
• Failed fusing heater lamp — the heater lamp(s) inside the fusing unit have burned out or degraded to the point where they cannot generate sufficient heat. This is the most common hardware cause of C3425 on older, high-mileage fusing units
• Blown thermal cutoff fuse (TCO) inside the fusing unit — a one-time-use safety fuse inside the fusing unit trips if the fuser ever overheats (from a previous C3722 event, for example). Once blown, the TCO permanently disconnects the heater circuit — the lamp receives no power and C3425 fires on the very next startup
• Faulty DCPU (DC Power Unit) — on DCPU-based models, a failing DCPU relay or triac does not deliver full AC power to the heater lamp, resulting in insufficient heat output. The lamp may be physically intact but receiving only partial power
• Faulty FUPU (Fusing Power Unit) — on C458/458e and C258/C308/C368, a failing FUPU performs the same failure mode as a faulty DCPU on other models
• Faulty IHPU (IH Power Unit) — on IH-fusing models (C558/C658/558e/658e, 654/754), a failing IHPU coil driver circuit cannot deliver sufficient induction heating energy to the fusing belt, causing a genuine warm-up failure
• Failed NF1 noise filter (654/754 series only) — a degraded noise filter NF1 in the AC supply path to the IHPU limits the available power to the induction heater, causing slow or incomplete warm-up
• Failed TEMS sensor — the TEMS reads falsely low even though the fuser is heating normally. The heater power supply is functioning correctly but the control board is receiving an underreading from the sensor and declaring a warm-up failure that is not actually occurring
• Faulty PRCB or MFPB — in rare cases the control board’s fusing heater control output or TEMS input circuit has failed
• Blown MFPB F12E fuse (558e/658e specific) — an inline fuse on the MFPB in the fusing heater control path can blow, completely disabling heater output on these models

Step-by-Step Troubleshooting Procedure

Step 1 — Full Power Cycle and Observation

Always start with a full power cycle. A transient voltage dropout during startup or a brief power interruption can occasionally cause the TEMS to miss its expected warm-up rate, triggering C3425 without a real hardware fault.

1. Power the machine fully OFF using the main power switch
2. Wait a full 60 seconds — do not simply press the power button immediately
3. During this wait, listen when you power back on — you should hear the fusing fan spin up and feel warmth near the right-side cover within the first 30 seconds if the heater is working
4. Power on and observe the warm-up sequence carefully:
   • If the machine begins warming up normally but C3425 appears after 60–90 seconds, this points to Trigger 2 (warm-up timeout) — the fuser is heating but not fast enough
   • If C3425 appears within the first few seconds before any warm-up time has elapsed, this points to a TEMS reading issue — likely a connector fault
   • If the machine warms up fully, declares ready, and then C3425 appears during or after the first print job, this points to Trigger 3 (post warm-up drop) — a marginal heater lamp or intermittent connector
5. If C3425 clears and does not return on subsequent startups, monitor the machine — a one-time occurrence may be a transient power event

Step 2 — Check the Right-Side Door and Interlock

1. Open and fully close the right-side door several times — confirm it swings freely and closes with a firm positive click
2. With the door fully closed, press on it gently — it should not flex inward or feel loose in the frame
3. Inspect the door interlock switch and its actuator for damage or wear — the switch should click audibly when pressed by hand
4. If the door does not latch firmly or the interlock switch is damaged, correct this before proceeding — on many bizhub models a partially open door directly disables the heater circuit

Step 3 — Reinstall the Fusing Unit and Reseat the Harness Connector

This single step resolves the majority of sudden-onset C3425 cases in the field.

1. Power OFF and unplug the machine — allow at least 15 minutes for the fusing unit to cool before handling
2. Open the right-side door and release the fusing unit locking lever
3. Pull the fusing unit out fully and set it aside on a flat, stable surface
4. Inspect the multi-pin connector on the fusing unit and its mating receptacle on the machine harness:
   • Look for bent or backed-out pins
   • Check for toner contamination on the contacts — clean with a dry cotton swab
   • Check for oxidation (green or white residue on the pins) — clean with electrical contact cleaner and allow to dry
   • Check for visible damage to the harness wires at any bend points near the connector
5. Push the fusing unit firmly back into its rails until the connector engages and the unit reaches full insertion depth
6. Re-engage the locking lever fully — an unsecured fusing unit can appear seated but have marginal connector contact
7. Close the right-side door, reconnect power, and power on
8. Observe the warm-up sequence — if the machine reaches ready status without C3425, run a test print to confirm stable operation

Step 4 — Check Fusing Temperature in Service Mode

Before replacing any parts, use service mode to observe the TEMS reading directly. This confirms within seconds whether the machine is genuinely not heating or whether a sensor is producing a false low 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 — typically found under Adjust → Fusing or I/O Check → Fusing
3. Observe the TEMS reading at ambient temperature (cold start):
   • A reading close to room temperature (e.g. 20–25°C) at cold start is correct — the sensor is working
   • A reading of 0°C or near-zero at cold start indicates the TEMS connector is open-circuit or the sensor has failed — the machine will never see the fuser heat up regardless of heater condition
   • A reading at maximum scale (e.g. 250°C+) at cold start indicates a shorted sensor or a wiring fault
4. After observing the cold-start reading, allow the machine to attempt warm-up while monitoring the TEMS:
   • If the TEMS reading does not rise at all during the warm-up attempt: the heater is not producing heat — suspect a failed heater lamp, blown TCO, or faulty heater power board (DCPU/FUPU/IHPU)
   • If the TEMS reading rises slowly but never reaches the target: the heater is producing reduced heat — suspect a weakened heater lamp or an underperforming heater power board
   • If the TEMS reading rises at a normal rate but then C3425 still fires: the sensor reading is crossing back below the threshold — possible intermittent connector or a marginal heater lamp that cannot sustain temperature under load

Step 5 — Inspect the Fusing Unit Heater Lamp and Internal TCO

If the service mode temperature monitor confirms that the TEMS is reading correctly but the fuser is not heating (reading does not rise during warm-up), the fault is inside the fusing unit — either the heater lamp or the thermal cutoff fuse.

1. With the fusing unit removed and fully cooled, open the fusing unit casing according to your service manual
2. Locate the heater lamp(s) — they resemble fluorescent tubes running the length of the fusing roller or belt. Most bizhub fusing units have two heater lamps — a main lamp and a sub lamp
3. Test each lamp for continuity using a multimeter in continuity mode:
   • Good lamp: shows continuity (near-zero resistance between the two end terminals)
   • Failed lamp: shows open circuit (no continuity)
   • Also inspect visually — a burned-out lamp often shows a blackened end or a broken filament visible through the glass
4. Locate the thermal cutoff fuse (TCO) — a small cylindrical component with two wire leads mounted near the heater lamp. Test for continuity:
   • Good TCO: shows continuity
   • Blown TCO: shows open circuit — the fusing unit must be replaced (the TCO is a one-time device and individual TCO replacement is not supported on most bizhub fusing units)
5. If the heater lamp is confirmed failed: on most bizhub models the lamp is available as a standalone replacement part. Replace the failed lamp(s) and test before replacing the complete fusing unit

Step 6 — Reseat and Check the DCPU, FUPU, or IHPU Connections

On all bizhub models, the heater power supply board (DCPU, FUPU, or IHPU) sits between the fusing unit and the main control board in the heater circuit. A loose connector at either junction in this path prevents full heater power from reaching the lamp or IH coil.

For DCPU models (C224/C284/C364 family, C454/C554, C452/C552/C652, C353/C451/C550/C650, C252/C300/C352):

1. Locate the DCPU board — typically mounted inside the right-side panel area
2. Reseat the connector between the fusing unit and the DCPU
3. Reseat the connector between the DCPU and the PRCB or MFPB
4. Inspect all pins for damage and oxidation

For FUPU models (C458/458e, C258/C308/C368):

1. Reseat: fusing unit → MFPB CN12E
2. Reseat: TEMS → relay → MFPB CN15E (TEMS connector path)
3. Also reseat all connectors on the FUPU board itself

For IHPU models (C558/C658/558e/658e, 654/754):

1. Reseat: fusing unit → MFPB CN22E (C558/C658/558e/658e) or fusing unit → IHPU → PRCB (654/754)
2. Reseat: TEMS/1 → relay CN109 → MFPB CN15E (C558/C658/558e/658e)
3. Also check all connectors between the IHPU and the IH coil inside the fusing unit
4. On 654/754 series: also check the NF1 (noise filter) connections — the NF1 is in the AC supply path to the IHPU and a loose connection here limits heater power

Step 7 — Check MFPB F12E Fuse (bizhub 558e / 658e)

On the bizhub 558e and 658e specifically, an inline fuse MFPB F12E sits in the fusing heater control path. A blown F12E completely disables heater output from the MFPB on these models — the IHPU receives no control signal and the fuser cannot heat.

1. Access the MFPB and locate fuse F12E — refer to the 558e/658e service manual for the exact location
2. Test F12E continuity with a multimeter — no continuity confirms F12E has blown
3. A blown F12E requires MFPB board-level repair or full MFPB replacement
4. Before replacing the MFPB, investigate what caused F12E to blow — a shorted fusing unit connector or a failed IHPU can cause the fuse to blow repeatedly

Step 8 — Replace the Fusing Unit

If the connector is confirmed good, the door closes correctly, the service mode temperature monitor shows the fuser is genuinely not heating, and the heater lamp and TCO inside the unit are confirmed failed — replace the fusing unit.

1. Order the correct fusing unit for your model and voltage — fusing units are voltage-specific (110V/120V or 220V/240V) and the part numbers differ between model generations. Never install a wrong-voltage fusing unit
2. Allow the old unit to cool fully before removal
3. Install the replacement unit, ensure the connector seats completely, and engage the locking lever
4. After installation, navigate to Service Mode → Counter and reset the fusing unit counter to zero
5. Perform the new-article release if C3461 appears alongside or after C3425 (see C3461 guide)
6. Allow the machine to complete a full warm-up cycle and confirm the TEMS reading in service mode reaches the correct operating temperature
7. Run a test print on several paper sizes and weights — confirm stable temperature throughout the print cycle

Step 9 — Replace the DCPU, FUPU, or IHPU

If a confirmed-good fusing unit still produces C3425 and the service mode temperature monitor shows the fuser is genuinely not heating, the heater power supply board is not delivering sufficient power to the heater elements.

• Replace DCPU — for bizhub C224/C284/C364 family, C454/C554, C452/C552/C652, C353/C451/C550/C650, C252/C300/C352
• Replace FUPU — for bizhub C458/458e, C258/C308/C368
• Replace IHPU — for bizhub C558/C658/558e/658e, 654/754

After replacing the heater power board, run a full warm-up cycle and monitor the TEMS reading in service mode to confirm the fuser now reaches operating temperature within the expected time.

Step 10 — Replace the NF1 Noise Filter (bizhub 654 / 754 Series Only)

On the bizhub 654/654e/754/754e, if IHPU replacement does not restore normal warm-up, replace the NF1 noise filter before attempting PRCB replacement. NF1 is in the AC supply path to the IHPU — a degraded NF1 limits the AC power available to the induction heating coil and can cause slow or incomplete warm-up that resists IHPU replacement.

Step 11 — Replace the PRCB or MFPB

Board replacement is the final step for C3425 and should only be considered after the fusing unit, heater power board (DCPU/FUPU/IHPU), and all connectors have been confirmed good with confirmed-good parts.

• PRCB — bizhub C224/C284/C364 family, C454/C554, 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

Quick Reference — Troubleshooting by Symptom

Understanding the C342x and C345x Fusing Heater Error Family

C3425 belongs to a closely related group of fusing heater errors. Understanding the family helps when multiple codes appear together:

• C3421 — Fusing heaters trouble: heating side (heater lamp / AC drive failure)
• C3423 — Fusing heaters trouble: pressurizing side
• C3424 — Fusing heaters trouble: soaking side (on C550/C650/C353 family)
• C3425 — Fusing warm-up trouble: TEMS does not reach target temperature in time
• C3451 — Fusing temperature failure (heating side low temperature) — thermistor-based detection
• C3452 — Fusing temperature failure (pressurizing side)

C3421 and C3425 appearing together almost always means a failed heater lamp or a blown TCO — C3421 detects the loss of AC heater current, while C3425 detects the resulting failure to reach temperature. Both clear when the fusing unit is replaced with a unit that has working lamps and an intact TCO.

C3425 appearing after C3722 (overtemperature) is a classic TCO failure sequence: the fuser overheated (C3722), the TCO tripped to protect the machine, and now the fuser cannot heat at all (C3425).

Preventing C3425 From Recurring

• Replace the fusing unit on schedule — heater lamps degrade gradually with use and page count. Most bizhub fusing units are rated for 150,000–300,000 pages depending on model. Running significantly beyond the rated life is the leading cause of heater lamp failure and C3425 on high-volume machines
• Never repeatedly reset C3722 without fixing the root cause — a C3722 that is repeatedly reset and ignored is the most common precursor to a blown TCO and subsequent C3425. Fix overtemperature errors before they trip the safety fuse
• Inspect the fusing unit connector at every PM visit — toner dust and thermal cycling degrade connector contact over time. A brief reseat during routine maintenance prevents the gradual connector degradation that causes Trigger 1 and Trigger 3 C3425 events
• Check the right-door latch during every PM — a door that does not close and latch fully disrupts heater control on many bizhub models. Worn door latches are inexpensive to replace and prevent unexpected C3425 calls
• Ensure stable power supply — machines on circuits with voltage drops, shared heavy equipment, or extension leads are prone to warm-up failures caused by insufficient heater lamp voltage. Verify the machine is on a dedicated, correctly rated circuit
• Always verify the voltage rating when ordering a replacement fusing unit — a wrong-voltage fusing unit produces the same C3425 symptom as a failed unit and is a common ordering error. Confirm 110V vs 220V from the machine’s serial plate before placing an order

Professional Technician Summary

Error Code C3425 on Konica Minolta bizhub machines is a fusing warm-up failure, and the diagnostic approach depends on when during the startup sequence C3425 fires.

C3425 within the first few seconds of startup is almost always a connector or TEMS sensor fault — the fuser physically cannot have failed its warm-up rate check that quickly from a cold start. Reseat the fusing unit and check the TEMS reading in service mode before touching anything else.

C3425 after 60–90 seconds is a genuine warm-up failure — the heater is either not working or working too weakly. Check heater lamp continuity and TCO continuity inside the fusing unit before replacing the complete unit or any boards.

C3425 following a previous C3722 event almost always means a blown TCO. Fix the original overtemperature cause (see C3722 guide), then replace the fusing unit.

On IH-fusing models (558e/658e, C558/C658, 654/754), the IHPU is more likely to be the cause of a genuine warm-up failure than on lamp-based models — IH power units fail in ways that reduce heating output without any other external signs, and they should be the next suspect after the fusing unit and connectors are confirmed good.