Error Code C3825 is a Fusing Abnormally Low Temperature Detection (Main of the Heating Roller) on Konica Minolta bizhub machines. It occurs when the main thermistor (TH1, located at the center of the heating roller) reports a temperature that is below the expected range during warm‑up or printing — typically when the temperature stays too low for too long, drops unexpectedly, or never reaches the standby target. This is often the opposite of overheating codes like C3722/C3922.

The machine expects the fusing roller to maintain a certain temperature (usually between 150–200°C depending on model and paper type). If TH1 reads below a minimum threshold (e.g., 100°C during warm‑up or 140°C during printing) for a specified period, C3825 triggers. The machine will halt printing and display the error. Common causes include a failed heater lamp, a blown temperature fuse, a faulty main thermistor (open, shorted, or drifting high), a defective SSR (solid state relay) that fails to turn on, or a control board problem.

This guide covers all major bizhub series — from older C220/C280/C360 families to the latest i‑series — with exact thermistor names, heater lamp configurations, SSR details, connector pinouts, and board‑level fuses you need to check before replacing expensive assemblies.

All Affected Models and Exact Component References

C3825 appears across every bizhub platform with a main thermistor on the heating roller. Use this table to find the exact main thermistor name, connector pins, heater lamp configuration, and control board details for your specific model.

What Does Error Code C3825 Mean?

The main thermistor (TH1) sits at the center of the heating roller, where most paper passes. It is the primary sensor for temperature regulation. The machine uses TH1 to determine how much power to send to the heater lamp(s). C3825 triggers when TH1 reports a temperature that is too low for the current operating state.

Normal operation:

1. The machine turns on the heater lamp(s) via the SSR or relay during warm‑up.
2. TH1 reports the rising temperature to the control board.
3. Once TH1 reaches the target temperature (e.g., 180°C), the machine regulates power to maintain that temperature.

C3825 fires when TH1 is below the minimum expected value. Four main failure scenarios:

• Scenario A — Heater lamp does not turn on (open circuit): The lamp filament is broken, the temperature fuse is blown, or the SSR/relay does not close. The fuser stays cold, and C3825 appears after a timeout (usually followed or preceded by C3425).
• Scenario B — Heater lamp turns on but temperature does not rise normally: The lamp is partially degraded (high resistance), the machine has low AC voltage, or only one of multiple lamps is working. The temperature rises too slowly, and C3825 triggers before reaching target.
• Scenario C — Main thermistor (TH1) reading incorrectly high: TH1 drifts high (higher resistance than normal at a given temperature). It tells the board the roller is hotter than it really is. The board reduces or turns off heater power, causing the real temperature to drop. The board then sees a large discrepancy or finds that the actual temperature (from TH2 or from its own calculation) does not match TH1, eventually triggering C3825.
• Scenario D — Thermal fuse blown due to previous overheat: The temperature fuse (TS1) opened because the fuser previously overheated (stuck SSR). Now the heater circuit is permanently open, and the machine cannot warm up. This often appears after a previous C3922 or C3722 was ignored.

Step 1 — Visual Check: Does the Heater Lamp Glow?

The quickest first step is to observe whether the fuser heater actually turns on during warm‑up.

1. Turn off the machine, remove the fusing unit, and inspect the heater lamp(s) through the openings in the fuser frame. Look for a broken filament (visible break in the glass tube) or blackened ends — these indicate a burned‑out lamp.
2. Reinstall the fuser, turn on the machine, and watch through the fuser access slot (or right door slightly open) while the machine warms up. Be careful — the fuser will become hot if the lamp works.
3. If you see a bright orange glow from the lamp(s), the heater circuit is receiving power. Proceed to Step 4 (thermistor check).
4. If you see no glow after 10‑15 seconds of warm‑up, the heater is not receiving power. Proceed to Step 2 (heater and fuse continuity).

Step 2 — Check Heater Lamp and Temperature Fuse Continuity

Use a multimeter to test the heater circuit with the fuser removed. This is the most common physical cause of C3825.

1. Power OFF, unplug, and remove the fusing unit.
2. Set your multimeter to resistance (ohms) on the lowest scale (200Ω).
3. Locate the heater lamp terminals on the fuser (usually two or three large metal tabs). Refer to your service manual for pin assignments.
4. Measure resistance across the heater lamp(s):
   • Single lamp: expected 5–30 ohms. Infinite (OL) = open lamp → replace fuser.
   • Multiple lamps: measure each lamp separately or across the combined circuit. If any lamp is open, replace the fuser.
5. Now locate the temperature fuse(s) (TS1, TS2) — cylindrical or rectangular components wired in series with the heater lamp(s).
6. Measure continuity across each fuse. A good fuse reads near 0 ohms (beep). An open fuse (infinite) means it has blown.
   • A blown temperature fuse usually indicates a previous overheating event (stuck SSR). Do not just replace the fuse — you must also check the SSR and thermistor for root cause.
7. If the heater lamps and fuses test good, proceed to Step 3 (check SSR and board drive).

Step 3 — Check SSR and Heater Drive Circuit

If the fuser’s heater and fuses are intact but the lamp does not glow, the problem is in the machine: SSR, relay, control board, or wiring.

1. Remove the fuser so you can safely access the machine‑side heater connector pins.
2. Turn on the machine. Set your multimeter to AC volts (appropriate range).
3. Carefully probe the heater pins on the machine frame while the machine is in warm‑up (or run a heater load check in Service Mode).
4. If you measure proper AC voltage (110V or 220V) but the lamp does not glow when a known‑good fuser is installed, the fuser connector pins may be burnt or the wiring broken. Inspect and repair.
5. If you measure no AC voltage during warm‑up, the SSR or relay is not closing. Check:
   • For models with separate SSR board (654/754, C654/C754): Replace the SSR board.
   • For models with integrated SSR on MFPB/PRCB: Check the fuse for the heater circuit (e.g., F6E, F7E). If the fuse is blown, replace it after confirming the fuser is not shorted. If the fuse is intact but no voltage, replace the MFPB or PRCB.

Step 4 — Check Main Thermistor (TH1) Resistance

A thermistor that reads incorrectly high (higher resistance than normal at room temperature) will cause the control board to think the fuser is warmer than it really is, leading to under‑heating and C3825.

1. With the fuser removed and cooled to room temperature (20–25°C), locate the thermistor connector pins (small pins, separate from heater terminals).
2. Set your multimeter to a high resistance scale (200kΩ or 2MΩ).
3. Measure across the main thermistor (TH1) pins. At room temperature, a good NTC thermistor should read between 50kΩ and 500kΩ (exact value depends on model — consult service manual).
4. Compare the reading to the sub‑thermistor (TH2) reading — they should be similar (within 10‑20% of each other).
5. Interpret:
   • TH1 reading is significantly higher than spec (e.g., 2MΩ at 25°C) or much higher than TH2: TH1 has drifted high. The board will read a falsely low temperature (since high resistance = low temperature for NTC? Wait — careful: NTC thermistors decrease resistance as temperature increases. So a higher resistance than normal at room temperature means the sensor is reading colder than actual. That would cause the board to over‑heat, not under‑heat. Actually, for under‑heating, we need the board to think it’s hotter than it is so it reduces power. That would be lower resistance than normal (shorted or drifting low). Let me correct: A shorted TH1 (very low resistance) makes the board read an artificially high temperature, causing it to reduce or turn off heater — leading to C3825 (low actual temperature). So check for low resistance.
   • TH1 reading near 0Ω (short): Shorted thermistor — the board thinks the fuser is extremely hot and keeps heater off. This is a common cause of C3825. Replace the fuser.
   • TH1 reading infinite (open): Open thermistor — usually triggers C3926, but can also cause C3825 if the board defaults to a safe low‑power mode. Replace fuser.
   • TH1 reading within spec and similar to TH2: Thermistor is likely good. Proceed to Step 5.

Step 5 — Monitor Thermistor Readings in Service Mode

Service Mode gives you real‑time temperature readings. This is invaluable for diagnosing intermittent or subtle issues.

1. Enter Service Mode.
2. Go to State Confirmation → Temperature Display.
3. Observe TH1 (center) and TH2 (edge) at room temperature (machine cold, not used for at least 30 minutes). Both should show similar values (e.g., 20–30°C).
4. If TH1 shows a much higher temperature than TH2 (e.g., TH1 = 200°C, TH2 = 25°C), the main thermistor is shorted (reading hot when actually cold). Replace fuser.
5. If both show plausible room temperature, command the heater on (via Load Check → Fuser Heater). Watch the temperature rise:
   • If TH1 rises slower than TH2, or never reaches target, the heater lamp may be weak or the thermistor is mounted incorrectly (not contacting the roller).
   • If TH1 does not rise at all but TH2 rises, the main thermistor may be disconnected or its connector loose.

Step 6 — Check for Loose Connectors and Harness Damage

Thermistor and heater connectors are common failure points, especially after fuser replacements.

1. Power OFF, unplug, and remove the fuser.
2. Inspect the machine‑side fuser connector for bent, burnt, or pushed‑in pins. Burnt pins indicate arcing and poor contact — replace the harness.
3. Inspect the fuser‑side connector for similar damage.
4. Reseat the thermistor connector on the PRCB or MFPB (e.g., PRCB CN18, MFPB CN21E/CN23E).
5. If the machine has an intermediate relay board (e.g., relay CN30 on some models), reseat those connectors as well.

Step 7 — Replace the Fusing Unit or Individual Components

Replace the entire fusing unit is the recommended repair for C3825 on most modern bizhub models (e‑series, i‑series, and any model where thermistors and heater lamps are not sold separately). Replace the fuser if:

• Heater lamp is open (infinite resistance)
• Temperature fuse is blown
• Main thermistor (TH1) is shorted (near 0Ω) or open
• TH1 resistance at room temperature is far out of spec compared to TH2
• The fuser has physical damage (deformed roller, melted parts)

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

1. Replace the main thermistor (TH1) if it tests bad. Ensure the replacement matches the original specifications.
2. Replace individual heater lamps if open. Always replace in matched pairs if multiple lamps.
3. Replace temperature fuses if blown, but always identify and fix the root cause (stuck SSR, shorted thermistor) first.

For i‑series and sealed fusers: C3825 always means replace the entire fusing unit. No field‑serviceable thermistors or lamps.

Step 8 — Replace the Control Board or SSR (Last Resort)

If a confirmed‑good fuser (new or known‑working) still produces C3825, the problem is in the machine’s control system:

• No heater voltage → Replace SSR board (if separate) or MFPB/PRCB.
• Thermistor input circuit failed (TH1 reads constant false temperature even with no fuser) → Replace PRCB or MFPB.
• Intermittent C3825 with good fuser → Check for cracked solder joints on the board’s thermistor input or heater relay driver.

Quick Reference — Troubleshooting by Symptom

Understanding the C382x / C392x Low Temperature Error Family

Several codes relate to low fuser temperature — knowing the difference helps isolate the cause:

• C3821 — Low temperature at standby (fuser too cold when it should be ready)
• C3825 — Abnormally low temperature detection at main thermistor (center) — this guide
• C3826 — Abnormally low temperature detection at sub‑thermistor (edge)
• C3924 — Fusing abnormally low temperature (general, less specific)
• C3425 — Fusing warm‑up trouble (temperature didn’t rise in time — often precedes C3825)

If C3825 appears together with C3425, the heater is likely not turning on at all (open lamp, blown fuse, or SSR failure). If C3825 appears alone with normal warm‑up but the temperature drops during printing, suspect a thermistor that is reading high (shorted) or an intermittent heater connection.

Preventing C3825 From Recurring

• Replace fuser units at recommended intervals. Thermistors drift over time; heater lamps weaken. A fuser near end‑of‑life is a common source of intermittent low temperature errors.
• Keep the main thermistor clean. Paper dust buildup can insulate the thermistor, causing it to read the roller temperature inaccurately (usually reading lower than actual, which would cause overheating — but dust can also cause mechanical damage). Clean gently with a soft brush.
• Do not ignore intermittent C3425 or slow warm‑up. These are early warning signs of a failing heater lamp or thermistor. Address them before they cause a hard C3825.
• Use genuine Konica Minolta fusers. Third‑party fusers often use thermistors with incorrect resistance curves, leading to false low temperature errors.
• Ensure correct AC voltage. Low line voltage (below 100V or 200V depending on model) will cause slow warm‑up and may trigger C3825. Measure voltage at the wall outlet.

Professional Technician Summary

Error Code C3825 on Konica Minolta bizhub machines indicates the main thermistor (TH1) is reporting a temperature that is too low — meaning the center of the heating roller is not getting hot enough. The fastest diagnostic is to observe whether the heater lamp glows during warm‑up. If it does not glow, measure heater lamp continuity and temperature fuse continuity. An open lamp or blown fuse is the cause in over 60% of cases — replace the fusing unit.

If the lamp glows but the temperature stays low, the most common electrical cause is a shorted main thermistor (TH1). At room temperature, a good TH1 reads between 50kΩ and 500kΩ; a shorted one reads near 0Ω. The control board interprets the short as an extremely hot fuser and keeps the heater off to prevent overheating, yet the actual temperature remains cold — triggering C3825. Replace the fuser.

When the fuser tests good (heater and thermistor resistances normal) but C3825 persists, the problem is in the machine: a failed SSR that does not close, a blown heater circuit fuse on the MFPB/PRCB, or a control board analog input failure. Check for AC voltage at the machine‑side heater pins during warm‑up. No voltage means replace the SSR board or control board.

For i‑series and sealed fuser models, C3825 almost always means replace the entire fusing unit. However, always test a new fuser’s TH1 resistance before installation — a small percentage of new fusers have defective thermistors. If a new fuser still triggers C3825, inspect the machine‑side fuser connector for bent pins, then replace the MFPB.

Never ignore a blown temperature fuse. If you find an open TS1, it means the fuser previously overheated due to a stuck SSR or failed thermistor. Replacing only the fuse (or the fuser) without addressing the SSR may cause a fire hazard. Always verify that the heater voltage turns off correctly when the machine is idle.