Error Code C392A is a Heating Roller Temperature Sensor Contamination (Main of the Heating Roller) on Konica Minolta bizhub machines. It occurs when the control board detects a persistent discrepancy between readings from the main thermistor (TH1) and the heating roller temperature sensor (TEMS/TEMS1). This code is unique because it directly flags contamination on the non-contact temperature sensor as the most likely cause, rather than a component failure.

Unlike codes like C3825 (low temperature) or C3722 (high temperature), C392A specifically indicates that the main thermistor and the temperature sensor are reporting significantly different values for an extended period. This typically means that the TEMS (a non‑contact sensor, often a thermopile or infrared sensor) has accumulated toner dust or paper debris, causing it to read incorrectly. The machine will continue to operate but may display this error and degrade print quality or eventually stop printing.

This guide covers all major bizhub series — from older C220/C280/C360 families to the latest i‑series — with exact sensor names, thermistor designations, connector pinouts, and board details you need to check. The first and most common fix is simply cleaning the TEMS sensor with a cotton swab.

All Affected Models and Exact Component References

C392A appears across nearly all bizhub platforms that use a non‑contact heating roller temperature sensor (TEMS) alongside a contact thermistor (TH1). Use this table to find the exact sensor name, thermistor designation, connector paths, and control board for your specific model. The sensor names vary significantly between model families.

What Does Error Code C392A Mean?

Modern Konica Minolta fusers use two types of temperature sensors working together:

• Thermistor (TH1, TH2, TH3, etc.): A contact sensor that physically touches the heating roller. It responds quickly to temperature changes but can wear or become dislodged.
• Temperature sensor (TEMS — Thermopile / Electro‑Mechanical Temperature Sensor or NC — Non‑Contact sensor): A non‑contact infrared sensor that measures the roller’s surface temperature from a distance. It does not wear physically but can become contaminated by toner dust, paper lint, or debris collecting on its lens.

Normal operation:

1. The machine uses both contact thermistors and the non‑contact TEMS to monitor fuser temperature.
2. The control board compares readings. They should track each other within a specified tolerance (usually 5–10°C).
3. If one sensor reads significantly different from the other for an extended period, the machine suspects one is faulty or contaminated.

C392A fires when the TEMS reading is not consistent with the main thermistor (TH1). Three main scenarios exist:

• Scenario A — TEMS lens contaminated (most common): Toner dust, paper lint, or silicone oil residue builds up on the TEMS lens. The infrared sensor cannot “see” the roller correctly, giving a false reading (usually too low or erratic). The thermistor reads correctly, but the discrepancy triggers C392A. This is the primary cause and is easily fixed by cleaning.
• Scenario B — Thermistor (TH1) drifting or failed: The contact thermistor may be dislodged from the roller, worn out, or reading incorrectly (e.g., shorted or open). The TEMS may be correct, but the thermistor is wrong. This is less common but possible.
• Scenario C — Board or harness failure (rare): The analog input circuit on the MFPB or PRCB for either sensor may have failed, or a connector in the path (e.g., relay CN109 to MFPB CN15E on some models) may be loose or damaged.[reference:18]

Step 1 — Clean the Heating Roller Temperature Sensor (TEMS)

This is the officially documented first step for C392A on every bizhub model — and it resolves the error in the majority of cases. The TEMS is typically a small sensor with a glass or plastic lens, located near the heating roller inside the fuser unit or on the machine frame looking at the roller.

1. Turn off and unplug the machine.
2. Open the right door and remove the fusing unit.
3. Locate the temperature sensor (TEMS/TEMS1/TEMS2/TEMS4). It is a small component (about 1–2 cm in size) with a transparent lens, usually mounted on a bracket near the heating roller, either on the fuser unit itself or on the machine frame.
4. Cleaning procedure:
   • Use a dry cotton swab to gently wipe the lens of the sensor. Remove any visible toner dust or debris.[reference:19][reference:20]
   • If dirt remains after dry wiping, dampen a fresh cotton swab with isopropyl alcohol (90% or higher) and gently wipe the lens. Then, use a dry cotton swab to wipe off any residue.[reference:21][reference:22]
   • Allow the sensor to air dry completely (about 30 seconds) before reinstalling the fuser.
5. While the fuser is removed, also inspect the heating roller surface for any contamination that could affect readings (e.g., toner buildup, paper stuck to the roller). Clean the roller gently if needed (using a soft, lint‑free cloth and fuser roller cleaner or alcohol).
6. Reinstall the fuser unit, close all covers, power on, and test.

Step 2 — Check the TEMS Installation Position and Connector

If cleaning does not resolve C392A, the sensor may be misaligned (not aimed at the roller correctly) or the connector may be loose.

1. With the fuser removed, visually check the TEMS mounting bracket. Ensure the sensor is securely fastened and aimed at the heating roller surface. A bent bracket may cause the sensor to read a cooler area (e.g., the roller edge or frame).
2. Locate the TEMS connector on the fuser or machine (varies by model). Reseat the connector firmly.
3. For models with an intermediate relay board (e.g., C658/C558: TEMS/1 → relay CN109 → MFPB CN15E), reseat both the relay connector and the MFPB end.[reference:23]
4. For models with IH fusers (C754/C654, 958/808), also check the connections between the IHPU (IH power supply) and the MFPB/PRCB, as the IH control circuit interacts with temperature readings.[reference:24][reference:25]
5. Power on and test.

Step 3 — Check the Main Thermistor (TH1) Resistance

If the TEMS is clean and connected properly, the issue may be with the main thermistor (TH1) that contacts the heating roller.

1. Power OFF, unplug, and remove the fusing unit. Allow it to cool to room temperature.
2. Set your multimeter to resistance (ohms) on a high scale (e.g., 200kΩ or 2MΩ).
3. Locate the thermistor pins on the fuser connector (refer to your service manual). Measure resistance across TH1.
   • At room temperature (20–25°C), a good NTC thermistor should read between approximately 50kΩ and 500kΩ. Exact values vary by model — consult your service manual.
   • If the reading is near 0Ω (shorted), infinite (open), or far out of specification, the thermistor has failed. Replace the fusing unit (or the thermistor if field‑replaceable).
4. If TH1 resistance is in spec, proceed to Step 4.

Step 4 — Perform the Sensor Monitor in Service Mode

Service Mode allows you to view both the thermistor and TEMS readings in real time, which is critical for determining which sensor is faulty.

1. Enter Service Mode (Utility → Details → Stop → 0 → 0 → Stop → 0 → 1, or model‑specific method).
2. Navigate to State Confirmation → Temperature Display (or Fuser Temperature Monitor).
3. Observe the reading for the main thermistor (TH1) and the heating roller temperature sensor (TEMS). At room temperature (fuser cold, not used for at least 30 minutes), both should show a similar temperature (e.g., 20–30°C).
4. If the readings are very different at room temperature:
   • TEMS reading is much lower than TH1 (e.g., TH1 = 25°C, TEMS = 5°C): TEMS lens contaminated or sensor failed. Clean again or replace fuser.
   • TH1 reading is much lower or higher than TEMS, and TH1 seems “stuck”: Thermistor likely failed or is dislodged from the roller. Replace fuser.
5. If both read similarly at room temperature, start a warm‑up cycle (or heater load check if available) and watch the readings. They should track together as the fuser heats up. A lagging or erratic TEMS reading confirms contamination.

Step 5 — Check Fuser Unit and Relay Harness Continuity

Intermittent C392A can be caused by broken wires or poor connections in the sensor path, especially on models with intermediate relay boards.

1. Power OFF and unplug the machine.
2. Locate the relay board or inline connectors in the TEMS path (e.g., relay CN109 on C658/C558 models).[reference:26]
3. Inspect the relay connector for bent or corroded pins. Reseat all connectors in the path.
4. Use a multimeter to check continuity from the TEMS connector (on the fuser) to the control board connector (MFPB CN15E or PRCB equivalent). A broken wire will show infinite resistance.
5. Repair or replace any damaged harness. On many models, the entire fuser unit must be replaced if the internal wiring is damaged.

Step 6 — Replace the Fusing Unit

If cleaning and connector reseating fail, and the thermistor tests good, the TEMS itself may be defective (not just contaminated). On most models, the TEMS is integrated into the fuser unit and cannot be replaced separately.

Replace the entire fusing unit if:

• The TEMS lens is clean but the sensor reading remains incorrect in Service Mode.
• The TEMS connector or internal wiring is damaged.
• The fuser unit is near or past its recommended life (thermistors wear out, heater lamps degrade, and the TEMS can fail).
• On i‑series and sealed fuser models, C392A after cleaning indicates a sealed sensor failure → replace fuser unit.[reference:27]

Step 7 — Check Board‑Level Fuses and Replace Control Board (Last Resort)

If a confirmed‑good fuser unit (new or known‑working) still produces C392A, the control board’s sensor input circuit has failed or a board fuse is blown.

For PRCB‑based models (C554/C454, C754/C654, C220/C280/C360):

1. Access the PRCB. Locate and test continuity of any fuses or ICPs associated with the thermistor/TEMS input circuit (refer to service manual).
2. If ICP/fuse is intact, replace the PRCB.[reference:28][reference:29]

For MFPB‑based models (C658/C558/C458, C368/C308/C258, 658e/558e, 958/808, i‑series):

1. Check the continuity of fuses like F12E on the MFPB (for C658/C558 models).[reference:30]
2. If the fuse is intact but the problem persists, replace the MFPB.[reference:31][reference:32]

For i‑series models (C450i, etc.): The control system includes CPUB and BASEB boards in addition to the MFPB. Check connections between the fusing unit‑relay CN3‑BASEB CN10E.[reference:33] If all connections are good, replace the fusing unit first, then CPUB, then BASEB as per the service manual flow.[reference:34]

Quick Reference — Troubleshooting by Symptom

Understanding the C392x Fuser Sensor Failure Family

C392A is one of several codes related to fuser sensor abnormalities. Knowing the family helps isolate the exact issue:

• C392A — Heating roller temperature sensor contamination (this guide) — typically a dirty TEMS lens or thermistor/TEMS discrepancy.
• C392B — Fusing sensor wire breaks detection (difference of temperature) — often indicates a broken wire or completely failed sensor, not just contamination.[reference:37]
• C3922 — Fusing sensor wire breaks detection (Edge of the heating roller) — broken or open thermistor at the edge.
• C3925 — Fusing sensor wire breaks detection (Main of the heating roller) — broken main thermistor or TEMS circuit.[reference:38]
• C3926 — Fusing sensor wire breaks detection (Center of the heating roller) — broken center thermistor.[reference:39]
• C3825/C3826 — Fusing abnormally low temperature detection — may be related if a contaminated TEMS reads too low.

If multiple C392x codes appear simultaneously, check common harnesses (e.g., the relay CN109 to MFPB CN15E path on C658/C558 models) that carry multiple sensor signals.[reference:40] A single loose connector can cause multiple sensor errors.

Preventing C392A From Recurring

• Keep the fuser area clean. Toner dust is the primary contaminant for the TEMS lens. During routine maintenance, use a vacuum cleaner with a narrow nozzle to remove toner dust from the fuser cavity.
• Use genuine Konica Minolta toner and paper. Third‑party toner tends to produce more airborne dust, which contaminates sensors faster. Low‑quality paper also generates more lint.
• Clean the TEMS lens at every PM visit. A quick wipe with a dry cotton swab takes 30 seconds and prevents C392A from ever occurring. For high‑volume machines, clean the TEMS every 50,000 pages.
• Do not ignore intermittent C392A. If the error appears occasionally, the TEMS is already partially contaminated. Clean it immediately, or it may lead to a hard failure.
• Replace fuser units at recommended intervals. A worn fuser may allow more toner dust to escape into the sensor area. A fresh fuser with clean sensors and good seals will reduce the risk of contamination.

Professional Technician Summary

Error Code C392A on Konica Minolta bizhub machines is one of the easiest errors to resolve — but only if you follow the correct procedure. The code directly indicates contamination of the heating roller temperature sensor (TEMS), a non‑contact infrared sensor. Do not replace the fuser unit or control board before cleaning the sensor.

In over 70% of cases, carefully wiping the TEMS lens with a dry cotton swab, followed by an alcohol‑dampened swab if needed, clears the error immediately. The entire cleaning procedure takes less than five minutes and requires no tools other than a swab and optionally a bottle of isopropyl alcohol.

If cleaning does not resolve C392A, check the TEMS connector and any intermediate relays (e.g., relay CN109 on C658/C558 models). A loose connection is the second most common cause. Reseating connectors often restores normal operation.

When cleaning and connector reseating fail, measure the main thermistor (TH1) resistance at room temperature. A shorted or open thermistor may be the culprit — replace the fusing unit. On models with IH fusers (C754/C654, 958/808), a failing IHPU or DCPU can also cause temperature discrepancies that trigger C392A; check these power supply units before replacing the control board.

For i‑series and other sealed fuser models, C392A after cleaning indicates a defective sealed sensor — replace the entire fusing unit. If a new fuser still triggers C392A, inspect the fuser‑relay‑BASEB harness and replace the CPUB then BASEB as a last resort.

Always reset the fuser life counter in Service Mode → Counter → Fuser Life after replacing the fusing unit. For models with thermistor or board replacement, consider performing a fuser temperature initialization (Service Mode → Fixing Adjustment → Thermistor Calibration) if available.