Error Code C3425 is a Fusing Warm-Up Trouble on Konica Minolta bizhub machines. It occurs when the fusing roller does not reach the target temperature within the specified time after the machine commands the fusing heater(s) to turn on. This is one of the most common fuser-related errors and can be caused by a failed heater lamp, a defective thermistor, a blown temperature fuse, a faulty solid state relay (SSR), or a control board failure.

The fusing unit must reach operating temperature (typically 160–200°C depending on media type) before the machine allows printing. If the temperature rises too slowly or not at all, C3425 fires. Unlike C3922/C3924 (which indicate temperature overshoot or abnormal readings), C3425 specifically means the fuser is not heating up as expected.

This guide covers all major bizhub series — from the older C220/C280/C360 family to the latest i-series — with exact heater lamp names, thermistor designations, temperature fuse ratings, connector pinouts, and board-level components you need to check before replacing expensive assemblies.

All Affected Models and Exact Component References

C3425 appears across every bizhub platform with a heated fusing unit. Use this table to find the exact heater lamps, thermistors, temperature fuses, and control board connections for your specific model. Guessing the wrong component wastes time and may damage the new fuser.

What Does Error Code C3425 Mean?

The fusing warm-up process is carefully timed and monitored by the control board. Here’s what happens during a normal warm-up:

1. The machine is powered on or exits sleep mode. The control board checks the initial fuser temperature via the main thermistor (TH1).
2. If the temperature is below the warm-up threshold (typically below 100°C), the board closes the heater relay or triggers the SSR to apply AC power to the heater lamp(s).
3. The heater lamp begins to glow, raising the temperature of the fusing roller.
4. The control board continuously monitors TH1 and TH2. It expects to see a steady rise in temperature (e.g., 5–10°C every few seconds).
5. Once the target warm-up temperature (usually around 160–200°C depending on model and paper type) is reached, the machine indicates “Ready to Print”.

C3425 fires when the temperature does not rise as expected within the allowed time. This is usually several seconds to a minute depending on the starting temperature. Four main failure scenarios:

• Scenario A — Heater lamp does not turn on at all: The lamp filament is open (burned out), the temperature fuse has blown, the SSR/relay has failed, or the control board is not sending the ON signal. The fuser stays cold and C3425 fires quickly.
• Scenario B — Heater lamp turns on but temperature rises too slowly: One of multiple lamps has failed (e.g., only H2 works but not H1), the lamp is partially degraded, or the thermistor is reading incorrectly (too high, so the board thinks it’s warmer than it actually is and reduces power).
• Scenario C — Thermistor reading is faulty: The fuser is actually heating normally, but the thermistor sends an incorrect low reading (or no reading) to the board. The board continues to apply power but never sees the temperature reach target — eventually C3425 may be accompanied by C3924 (abnormally low temperature) after a timeout.
• Scenario D — AC power supply problem: The machine is not receiving proper AC voltage, or the heater circuit wiring has an open connection (loose connector, broken wire, damaged fuser connector pin).

Step 1 — Check for Loose Fuser Connectors and AC Supply

Before replacing any parts, verify that the fuser unit is fully seated and that the machine is receiving proper AC voltage.

1. Turn off and unplug the machine.
2. Remove and reinsert the fusing unit firmly — a partially seated fuser may not make proper heater or thermistor contact.
3. Inspect the fuser connector pins (on the fuser and on the machine frame) for bent, burnt, or pushed-in pins. Burnt pins indicate a poor connection that caused arcing — replace the fuser and possibly the machine-side harness.
4. Plug the machine into a known-good wall outlet. Measure AC voltage at the outlet — should be within spec (110–120V or 220–240V depending on region). Low voltage can cause slow warm-up and trigger C3425.
5. If the machine is plugged into a power strip or UPS, try direct connection to wall outlet — some power conditioners can cause heater misfires.
6. Power on and monitor. If C3425 persists, proceed to Step 2.

Step 2 — Check the Fuser Heater Lamp Continuity (Resistance Test)

The heater lamp is the most common failed component in a fuser that will not warm up. Testing it takes two minutes and requires only a multimeter.

1. Power OFF, unplug, and remove the fusing unit.
2. Locate the heater lamp terminals on the fuser — usually two or three large metal tabs or pins on the end of the fuser (often covered by a plastic cap that can be removed). Refer to your model’s service manual for exact heater pin locations.
3. Set your multimeter to resistance (ohms) on the lowest scale (200 ohms).
4. Probe the heater lamp terminals. Expected resistance:
   • Single lamp: typically 5–30 ohms (depending on wattage and voltage)
   • Two lamps in parallel: each lamp may measure 10-60 ohms, combined reading lower
   • If the reading is infinite (OL or open circuit), the heater lamp filament has burned out. Replace the fusing unit (or the heater lamp if field-replaceable on your model — some older models allow lamp replacement).
   • If the reading is very low (less than 1 ohm), the lamp may be shorted — also replace.
5. If the heater lamp tests good, proceed to Step 3.

Step 3 — Check the Temperature Fuse(s) Continuity

The temperature fuse is a one-time thermal cutoff that blows if the fuser exceeds a critical temperature (typically 240–260°C). Once blown, it must be replaced — you cannot reset it. A blown fuse will cause C3425 because the heater circuit is open.

1. With the fuser removed, locate the temperature fuse(s) — small cylindrical or rectangular components, usually white or black, marked with a temperature rating (e.g., 240°C, TF 250°C). They are often wired in series with the heater lamp.
2. Set your multimeter to continuity (beep mode) or lowest resistance scale.
3. Probe the two leads of the temperature fuse. A good fuse reads near 0 ohms (or beeps). An open (infinite) fuse means it has blown.
4. A blown temperature fuse almost always indicates the fuser previously overheated due to a stuck SSR, failed thermistor, or other control problem. Do not simply replace the fuse — you must also diagnose and fix the root cause, or the new fuse will blow again immediately. In practice, on most modern bizhub models, the temperature fuse is not field-replaceable — you replace the entire fusing unit.
5. If the fuse tests good, proceed to Step 4.

Step 4 — Check Thermistor Resistance (Fuser Removed)

Thermistors are temperature-sensing resistors that change resistance with heat. At room temperature (25°C / 77°F), a good thermistor typically reads between 50kΩ and 500kΩ, depending on the type (NTC — negative temperature coefficient). If the thermistor is open, shorted, or reads far out of spec, the control board cannot read the temperature correctly.

1. With the fuser removed and cooled to room temperature, locate the thermistor connector pins on the fuser (usually 2–4 small pins separate from the heater pins).
2. Set your multimeter to a high resistance scale (e.g., 200kΩ or 2MΩ).
3. Measure across the main thermistor (TH1) pins. At room temperature (20–25°C), you should see a reading between approximately 50kΩ and 500kΩ. Exact values vary by model — consult your service manual for the thermistor type and resistance vs. temperature table.
4. If the reading is infinite (open) or 0 (short), the thermistor is defective — replace the fuser unit.
5. If the reading is within range, you can also test the sub-thermistor (TH2) similarly.
6. If the thermistor reads correctly, the problem is likely in the heater power supply (SSR/relay or control board). Proceed to Step 5.

Step 5 — Perform the Heater Lamp Operation Check in Service Mode (Load Check)

Service Mode allows you to command the heater lamps to turn on and monitor the thermistor readings in real time. Be extremely careful — the fuser will become very hot during this test.

1. Enter Service Mode.
2. Navigate to State Confirmation → Load Check (or Fuser Heater Check).
3. Select Fuser Heater (Main/Sub) or individual heater lamp control.
4. Command the heater ON while watching the thermistor temperature reading in Service Mode (usually in the Sensor Check or Temperature Display section).
5. Interpret results:
   • Heater turns on (you can see the lamp glow through the fuser opening) and temperature rises steadily: The fuser is working electrically. C3425 may be caused by a dirty thermistor (paper dust insulating it) or an intermittent connection. Clean the thermistor gently with a soft brush and compressed air.
   • Heater does not glow, but thermistor reading is normal room temperature: No power reaching heater. Proceed to Step 6 (check SSR and board).
   • Heater glows but temperature rises very slowly or not at all: Multiple heater lamps — one may be burned out. Check resistance of each lamp separately. Or the thermistor is reading incorrectly high (so board thinks it’s warmer and reduces power).
   • Heater glows and temperature reading jumps erratically or shows very high values (e.g., 300°C) instantly: Thermistor shorted or connector problem — replace fuser.

Step 6 — Check SSR (Solid State Relay) and Heater Drive Circuit

If the heater lamp tests good, the temperature fuse is intact, and the thermistor reads correctly, but the lamp does not glow in the load check, the problem is in the SSR or the control board’s heater relay drive circuit.

For PRCB-based models with separate heater relays (C224/C284 family, C220 family):

1. Access the PRCB and locate the heater relay(s) — usually labeled RL1 and RL2 near connector CN17/CN18.
2. With the machine on and in the heater load check, listen for the relay clicking. If you hear a click but the lamp does not glow, the relay contacts may be burned out — replace the PRCB.
3. If no click, check the drive signal from the PRCB to the relay coil — this requires a multimeter and service manual pinout. If the drive signal is present but relay does not activate, replace the relay (board-level repair) or replace the PRCB.

For MFPB-based models with integrated SSR (227/287/367, C258/C308/C368, 458e series):

1. The SSR is a solid-state device on the MFPB. It is not field-repairable.
2. Check for the presence of AC voltage at the fuser heater connector (on the machine frame) during the load check. If AC voltage is present (use a multimeter set to AC volts, carefully probing the heater pins), the SSR is good and the problem is in the fuser itself (open heater or fuse). If no AC voltage, the SSR or its drive circuit has failed — replace the MFPB.
3. Also check the fuse associated with the heater circuit on the MFPB (e.g., F6E, F7E). A blown fuse indicates an overcurrent condition — often a shorted heater lamp or a fuser that drew too much current.

For models with a separate external SSR board (654/754, C654/C754):

1. Locate the SSR board (usually near the power supply).
2. Check for input signal (DC voltage from PRCB) and output AC voltage to the fuser. Replace the SSR board if input is present but output is missing.

Step 7 — Replace the Fusing Unit or Heater Components

Replace the entire fusing unit is the recommended repair for C3425 on most modern bizhub models (e-series, i-series, and any model where heater lamps and temperature fuses are not sold separately). Replace the fuser if:

• Heater lamp tests open (no continuity)
• Temperature fuse tests open
• Thermistor tests out of spec
• The fuser has exceeded its rated life and is failing intermittently

For older models (C220/C280/C360, 223/283/363, 652/552/452 series, 654/754) where heater lamps and fuses are available as separate parts:


1. You can replace the heater lamp(s) individually. Ensure you use the correct wattage and voltage rating.
2. Replace the temperature fuse if blown, but always investigate why it blew (stuck SSR, failed thermistor) to prevent recurrence.
3. After replacing components, warm up the machine and monitor temperatures in Service Mode.

Step 8 — Replace the Control Board (Last Resort)

If a confirmed-good fuser (new or known-working) still produces C3425 and the SSR/heater drive signals are absent, replace the control board:

• Replace PRCB — for C224/C284/C364 family, C220/C280/C360, 654/754, 223/283/363/423, C652/C552/C452
• Replace MFPB — for 227/287/367, C258/C308/C368, C458/C558/C658, 458e/558e/658e, i-series
• Replace SSR board — for models with a separate SSR board (654/754, C654/C754) before replacing the PRCB
ℹ️ After fuser or board replacement: Always reset the fuser life counter in Service Mode → Counter → Fuser Life. Also run the fuser temperature stabilization routine if available in Service Mode → Fixing Adjustment → Fuser Warm-Up.

Quick Reference — Troubleshooting by Symptom

Symptom	Most Likely Cause	First Action
C3425 immediately after power on, fuser cold	Open heater lamp or blown temperature fuse	Remove fuser, measure heater resistance (should be 5–30Ω); check temperature fuse continuity
C3425 after a fuser replacement (new fuser)	New fuser defective or machine-side SSR/board issue	Test heater continuity on new fuser; if good, check AC voltage at fuser connector during warm-up
C3425 intermittent, especially on cold mornings	Heater lamp aged (high resistance) or thermistor coated with paper dust	Clean thermistor with soft brush; measure heater resistance cold vs hot; replace fuser if out of spec
Fuser heats slowly, C3425 after long warm-up time	One of multiple heater lamps failed, or low AC line voltage	Check resistance of each lamp separately; measure wall outlet voltage
Heater glows in load check but C3425 still occurs	Thermistor reading incorrectly (too high) or thermistor connector loose	Check thermistor resistance at room temperature; reseat thermistor connector on board and fuser
Heater does not glow, no AC voltage at fuser connector	Blown SSR/heater fuse on board, or failed SSR/relay	Check continuity of heater circuit fuse (F6E/F7E); if fuse intact, replace MFPB/PRCB or SSR board
Heater glows but temperature reading jumps instantly to 300°C+	Shorted thermistor or thermistor connector shorted	Replace fuser unit (thermistor not serviceable separately on most models)
C3425 on i-series (C450i, etc.) with new fuser	MFPB heater driver failed, or fuser connector pins damaged	Inspect fuser connector on machine side for burnt pins; replace MFPB if pins are good but no heater AC output

Understanding the C342x / C392x Fuser Temperature Error Family

C3425 is one of several fuser temperature-related codes. Knowing the difference helps pinpoint the cause:

• C3421 — Fusing warm-up failure (no temperature rise at all — usually heater or fuse open)
• C3423 — Fusing warm-up failure (temperature rise too slow — one lamp out or thermistor drift)
• C3425 — Fusing warm-up trouble (generic warm-up timeout — most common code)
• C3922 — Fusing temperature abnormal rise (overshoot — usually stuck SSR or thermistor reading low)
• C3924 — Fusing temperature abnormal low (heater not getting power or thermistor reading high)
• C3926 — Fusing temperature sensor abnormality (thermistor open or short)

If C3425 appears together with C3924, the heater is likely not receiving power (open lamp, blown fuse, or SSR failure). If C3425 appears together with C3922, the thermistor is likely reading incorrectly low, causing the board to apply full power even when the fuser is actually hot.

Preventing C3425 From Recurring

• Replace fuser units at or before their recommended life. Heater lamps degrade over time, becoming higher resistance and requiring more time to warm up. A fuser near end of life is a common cause of intermittent C3425 on cold mornings.
• Never bypass the temperature fuse. Some technicians try to short out a blown temperature fuse to get the machine working — this is extremely dangerous and can cause a fuser fire. Always replace the fuse (or the entire fuser).
• Keep the thermistors clean. Paper dust buildup on the thermistor insulates it, causing inaccurate readings. Clean the thermistors gently with a soft brush at every PM visit.
• Ensure proper AC voltage and stable power. Frequent brownouts or low line voltage can cause heater lamps to run cold and eventually fail prematurely. Install a line conditioner if the site has unstable power.
• Do not ignore fuser life warnings. When the machine displays “Fuser life end” or similar, replace the fuser proactively. Continuing to run a worn fuser stresses the heater lamps and SSR.

Professional Technician Summary

Error Code C3425 on Konica Minolta bizhub machines indicates the fuser is not warming up within the expected time. The fastest diagnostic is to remove the fuser and measure heater lamp resistance with a multimeter. An open lamp (infinite ohms) is the cause in over 60% of cases, especially if the fuser has been in use for more than its rated life.

In office environments, the most common cause is a failed heater lamp due to normal wear. Replace the fusing unit — the job takes 10 minutes and resolves C3425 immediately. Always test the new fuser’s heater continuity before installation (some new fusers are defective from the factory).

When the heater lamp tests good, check the temperature fuse continuity. A blown fuse indicates the fuser previously overheated — do not just replace the fuse; also inspect the SSR and thermistor for the root cause. On most modern bizhub models, the entire fuser is replaced, so this is a non-issue.

If the fuser is good (heater resistance normal, fuse intact, thermistor resistance within spec at room temperature) but the lamp still does not glow, check for AC voltage at the fuser connector during warm-up. No AC voltage points to a failed SSR, heater relay, or control board. Replace the MFPB or PRCB as a last resort after eliminating all other causes.

For the i-series and other sealed fuser models, C3425 almost always means replace the fusing unit. These fusers are not serviceable at the component level. If a new fuser still triggers C3425, inspect the fuser connector on the machine side for burnt pins, then replace the MFPB.