Error Code C3722 is a Fusing Abnormally High Temperature Detection (Edge of the Heating Roller) on Konica Minolta bizhub machines. It occurs when the sub‑thermistor (also called the edge thermistor or TH2) detects a temperature at the end of the heating roller that exceeds the machine’s safety threshold — typically above 250°C or a rate of rise that is too fast. This is a critical safety error designed to prevent fuser fires and roller damage.

Unlike C3922 (general abnormal temperature rise), C3722 specifically points to uneven heating across the roller width — the edge is significantly hotter than the center or the sub‑thermistor reading is faulty. The machine will immediately cut power to the heater lamps and display C3722. The error can be caused by a shorted or drifting sub‑thermistor, a stuck solid state relay (SSR) that keeps the heater on continuously, a broken heater lamp that is heating unevenly, or a control board failure.

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

C3722 appears across every bizhub platform with dual thermistors (center and edge) on the heating roller. Use this table to find the exact sub‑thermistor name, connector pins, heater lamp configuration, and control board details for your specific model.

What Does Error Code C3722 Mean?

The fusing unit has two thermistors on the heating roller: TH1 (center) monitors the middle of the roller where paper passes, and TH2 (edge/sub) monitors the end of the roller where no paper contacts. The edge normally runs slightly cooler because there is less heat demand. C3722 triggers when TH2 reports a temperature above the safety threshold (typically 240–260°C depending on model) or when TH2 temperature rises faster than physically possible (indicating a sensor fault).

Normal operation:

1. The control board turns on the heater lamp(s) via the SSR or relay during warm‑up or printing.
2. TH1 and TH2 continuously report temperatures.
3. The board regulates power to keep the center at target (e.g., 180°C) and ensures the edge does not exceed a safe limit.

C3722 fires when the edge temperature is reported as too high. Four main failure scenarios:

• Scenario A — Shorted or low‑resistance sub‑thermistor (TH2): NTC thermistors decrease resistance as temperature rises. If TH2 is internally shorted (very low resistance even at room temperature), the control board reads an extremely high temperature (e.g., 300°C) even though the roller is cold. This triggers C3722 instantly on power‑up.
• Scenario B — Stuck SSR or welded heater relay: The SSR or relay fails in the closed position, applying continuous AC power to the heater lamp(s). The fuser overheats rapidly, and TH2 (at the edge) may exceed the threshold before TH1 even reaches target. The machine will trip C3722 and cut power, but the fuser will remain dangerously hot.
• Scenario C — Broken heater lamp causing uneven heating: If one of multiple heater lamps (e.g., the end lamp) is broken, the remaining lamp may be positioned such that it heats the edge more than the center. TH2 rises faster than normal and can trigger C3722 during warm‑up.
• Scenario D — Thermistor contamination or physical damage: Paper dust buildup on TH2 can insulate it and cause erratic readings, but this usually leads to low temperature readings (C3924) rather than high. However, if the thermistor is crushed or its wires are pinched, it may read shorted and cause C3722.

Step 1 — Immediate Safety Check: Is the Fuser Still Hot?

Before any diagnostics, ensure the fuser is not actually overheating. A stuck SSR can keep the heater on even after the machine powers off.

1. Turn off the main power switch and unplug the machine.
2. Carefully open the right door and remove the fusing unit. Use a non‑contact infrared thermometer to check the surface temperature of the heating roller. If it is above 100°C, allow it to cool completely (30 minutes) before handling.
3. If the fuser remains hot more than 10 minutes after power‑off, the SSR or relay is likely welded in the closed position. Do not reinstall that fuser until the SSR is replaced.
4. With the fuser removed, visually inspect the heating roller surface. Look for melted toner, roller damage, or a dark spot at the edge — these are signs of actual overheating (Scenario B).
5. If the fuser is cool to the touch but C3722 appears immediately on power‑up, proceed to Step 2 (thermistor resistance test).

Step 2 — Check Sub‑Thermistor (TH2) Resistance (Fuser Removed)

A shorted or out‑of‑spec TH2 is the most common cause of C3722 on a cold machine. Testing takes two minutes with a multimeter.

1. Power OFF, unplug, and remove the fusing unit. Allow it to cool to room temperature (around 20–25°C).
2. Locate the thermistor connector pins on the fuser — usually 2–4 small pins separate from the heater terminals. Refer to your model’s row in the table above for the connector mapping.
3. Set your multimeter to resistance (ohms) on a scale that can read up to 500kΩ (e.g., 200kΩ or 2MΩ).
4. Measure across the sub‑thermistor (TH2) pins. At room temperature (20–25°C), a good NTC thermistor typically reads between 50kΩ and 500kΩ. Exact values vary by model — consult your service manual for the specific thermistor type.
5. Interpret the reading:
   • Reading near 0Ω (short circuit): TH2 is shorted. Replace the fusing unit (or thermistor if field‑replaceable).
   • Reading within expected range (e.g., 100kΩ at 25°C): TH2 is electrically good. Proceed to Step 3.
   • Reading infinite (open circuit): TH2 is broken — this usually triggers C3926 (thermistor abnormality) but can also cause C3722 if the board interprets the open as an extreme temperature. Replace fuser.
   • Reading significantly lower than spec (e.g., 5kΩ at 25°C): TH2 is drifting low and will read too hot even at normal temperatures. Replace fuser.
6. For comparison, also measure the main thermistor (TH1) resistance — it should be similar to TH2 (within 10–20%). A large difference between TH1 and TH2 at room temperature indicates one thermistor has drifted.

Step 3 — Check for Stuck SSR / Welded Heater Relay

If the thermistor tests good, the most likely cause is the SSR or heater relay stuck in the ON position, causing the fuser to overheat continuously.

Test with fuser removed and machine powered on (be extremely careful — high voltage):

1. Remove the fusing unit so you can access the heater voltage pins on the machine side.
2. Turn on the machine (without the fuser installed). The machine will likely show an open fuser error — ignore it for now.
3. Set your multimeter to AC volts (appropriate range, e.g., 200V or 600V). Carefully probe the heater lamp connector pins on the machine frame (where the fuser would plug in).
4. If you measure significant AC voltage (e.g., 110V or 220V) at the heater pins while the machine is idle (not in a warm‑up cycle), the SSR or relay is shorted. Do not install a new fuser until the SSR/relay is replaced.
5. If no voltage is present, enter Service Mode and start a heater load check (as described in Step 5 of the C3425 guide). The voltage should appear only when the heater is commanded on. If the voltage stays on even after the load check is stopped, the SSR is stuck.

For models with external SSR boards (654/754, C654/C754): The SSR board can be replaced separately. For models with integrated SSR on MFPB/PRCB, the entire control board must be replaced if the SSR is shorted.

Step 4 — Inspect Fuser Connector Pins for Damage

Burnt or bent pins in the fuser connector can cause incorrect thermistor readings or intermittent heater control, leading to C3722.

1. With the fuser removed, inspect the machine‑side fuser connector (typically a multi‑pin plastic housing). Look for:
   • Burnt or blackened pins (indicates arcing from poor contact)
   • Bent pins that may not make contact with the fuser
   • Pushed‑in pins (recessed too deep to touch the fuser)
2. Inspect the fuser‑side connector for similar damage.
3. If any pins are burnt, the connector must be replaced (machine‑side harness) and the fuser should also be replaced (since the fuser connector is usually integral to the fuser).
4. Clean any toner or debris from the connector using compressed air and a small brush.

Step 5 — Perform Thermistor Monitor in Service Mode (Temperature Display)

Service Mode allows you to view real‑time thermistor readings. This is critical to determine whether the problem is a sensor fault or actual overheating.

1. Enter Service Mode.
2. Navigate to State Confirmation → Temperature Display (or Fuser Temperature Monitor).
3. You will see readings for TH1 (center) and TH2 (edge).
4. With the machine cold (room temperature, not having printed for at least 30 minutes):
   • Both TH1 and TH2 should read about the same (e.g., 20–30°C).
   • If TH2 reads significantly higher than TH1 (e.g., TH1 = 25°C, TH2 = 250°C), the sub‑thermistor is shorted or the connector is faulty.
   • If TH2 reads a plausible temperature (e.g., within 5–10°C of TH1), the thermistor is likely good — the problem is actual overheating.
5. Command a heater load check (Heater ON) while watching the temperature display:
   • If TH1 and TH2 rise together but TH2 quickly exceeds 250°C while TH1 is still below 200°C, you have uneven heating (broken end lamp or heating element).
   • If both temperatures rise normally and TH2 stays within safe limits, the error may have been transient. Clear the code and run a test print.

Step 6 — Check Heater Lamp Continuity and Configuration

On models with multiple heater lamps (e.g., H1 center, H2 edge), a failed lamp can cause uneven heating that triggers C3722.

1. Remove the fuser and measure resistance across each heater lamp separately (refer to service manual for pin assignments).
2. Expected resistance: typically 5–30 ohms per lamp at room temperature. Infinite resistance = open lamp.
3. If the edge lamp (H2) is open, the center lamp may over‑heat the edge indirectly, but this is rare. More commonly, if the center lamp fails, the end lamp alone may overheat the edge because the thermostat/SSR tries to compensate.
4. If one lamp is open, replace the entire fusing unit (or replace the individual lamp if your model supports it).

Step 7 — Replace the Sub‑Thermistor or Fusing Unit

Replace the entire fusing unit is the recommended repair for C3722 on most modern bizhub models (e‑series, i‑series, and any model where thermistors are not sold separately). Replace the fuser if:

• TH2 resistance is shorted or significantly out of spec at room temperature
• TH2 shows a temperature difference of more than 10°C from TH1 at cold start
• The fuser has actual physical damage (scorched roller, melted toner)
• A heater lamp is open

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

1. You can replace the sub‑thermistor (TH2) individually. Ensure the replacement matches the original resistance vs. temperature curve.
2. After replacement, perform a thermistor calibration in Service Mode if available (Fuser Temperature Adjustment).

For i‑series and sealed fusers: C3722 always means replace the entire fusing unit. Thermistors are not field‑serviceable.

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

If a confirmed‑good fuser (new or known‑working) still produces C3722 and the thermistor readings are normal at cold start, the problem is in the SSR or control board:

• Stuck SSR (welded): On models with a separate SSR board (654/754, C654/C754), replace the SSR board first. On models with integrated SSR (most MFPB/PRCB), replace the entire MFPB or PRCB.
• Control board analog input failure: If TH2 reads a constant false high temperature even with no fuser installed (open circuit on the thermistor input), the board’s analog‑to‑digital converter for TH2 has failed. Replace the PRCB or MFPB.

Quick Reference — Troubleshooting by Symptom

Understanding the C372x / C392x High Temperature Error Family

Several codes relate to fuser overheating — knowing the difference pinpoints the exact cause:

• C3721 — Abnormally high temperature at center (main thermistor TH1) — indicates general overheating, often a stuck SSR.
• C3722 — Abnormally high temperature at edge (sub‑thermistor TH2) — this guide. Often a shorted TH2 or uneven heating.
• C3723 — Abnormally high temperature at pressure roller — pressure roller thermistor (if equipped).
• C3922 — Fusing temperature abnormal rise (general) — less specific, may be caused by slow SSR response or thermistor drift.
• C3926 — Fusing temperature sensor abnormality — open thermistor (usually triggers C3926, not C3722).

If C3722 appears together with any of the above, the root cause is almost always a shorted SSR or a failed thermistor. Do not clear the code and continue — diagnose immediately.

Preventing C3722 From Recurring

• Replace fuser units at recommended intervals. Thermistors degrade over time and can drift low, causing false high temperature readings. A fuser near end‑of‑life is a common source of intermittent C3722.
• Keep thermistors clean. Paper dust buildup on the thermistor can cause insulation and erratic readings. Gently clean TH1 and TH2 with a soft brush at every PM visit.
• Do not ignore other fuser codes. If you see intermittent C3922 or C3924, address them early before the SSR or thermistors fail completely.
• Use genuine Konica Minolta fusers. Third‑party fusers often use lower‑quality thermistors that drift out of spec quickly, leading to false C3722 errors.
• Check AC line voltage stability. Frequent brownouts or surges can damage the SSR and cause it to fail shorted. Install a line conditioner if the site has unstable power.

Professional Technician Summary

Error Code C3722 on Konica Minolta bizhub machines indicates the edge of the heating roller is reported as dangerously hot. The fastest diagnostic is to measure the sub‑thermistor (TH2) resistance at room temperature with the fuser removed. A shorted TH2 (near 0Ω) is the cause in over 50% of cases, especially if the error appears immediately on cold start. Replace the fusing unit — the job takes 10 minutes and resolves the issue.

If TH2 resistance is normal, the next most common cause is a stuck SSR or welded heater relay. Check for AC voltage at the heater pins when the machine is idle. If voltage is present, the SSR is shorted — this is a fire hazard. Unplug the machine immediately and replace the SSR board or the entire control board (MFPB/PRCB) before reinstalling any fuser.

For models with multiple heater lamps, a broken end lamp can cause uneven heating that triggers C3722 only after the machine runs for several minutes. Measure each lamp’s resistance with the fuser removed — an open lamp means replace the fuser.

On i‑series and other sealed fuser models, C3722 is almost always a fuser replacement code. However, always test the new fuser’s TH2 resistance before installation — a small percentage of new fusers have defective thermistors. If a new fuser still triggers C3722, inspect the machine‑side fuser connector for bent or burnt pins, then replace the MFPB.

Never clear C3722 and continue printing without diagnosing. A true overheating condition can melt the fuser roller, damage the pressure roller, and in extreme cases, cause a fire. Always verify the fuser actually cools down and that the heater voltage turns off correctly before returning the machine to service.