P25CB

Reductant Heater Control Circuit High Bank 1 Heater 2

P25CB is a generic OBD-II powertrain diagnostic trouble code: Reductant Heater Control Circuit High Bank 1 Heater 2. It is logged by the engine control unit when the powertrain monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.

Code
P25CB
Group
Powertrain
System
Powertrain
Severity
Warning (MIL on)
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What P25CB means

P25CB indicates that the powertrain or aftertreatment control module has detected a high voltage or open-load condition on the control circuit for the Bank 1 Heater 2 element in the DEF (reductant) heating system. DEF systems use electric heaters to thaw frozen fluid in the tank, lines, and pump in cold climates, ensuring the system can dose at low ambient temperatures.

A high circuit condition typically means the control module is commanding the heater off but seeing unexpectedly high voltage on the circuit, or that an open circuit has been detected in the heater element or driver circuitry. This can result from a broken heater element, a severed or corroded heater control wire, a failed relay or driver module, or an internal PCM fault.

While this fault does not directly disable the engine, it leaves the DEF system without adequate cold-weather protection. In freezing conditions, the DEF fluid may solidify, causing additional SCR system faults and eventual torque de-rate if the system cannot dose. Diagnosis should proceed with a circuit resistance and voltage check before condemning the heater element or control module.

Common causes

Most-frequently reported root causes when P25CB is logged.

  • 1
    Open circuit in the Bank 1 Heater 2 element wiring.
  • 2
    Failed or burnt-out DEF heater element.
  • 3
    Damaged or corroded heater circuit connector.
  • 4
    Faulty heater relay or solid-state driver remaining open.
  • 5
    PCM or aftertreatment module internal heater driver fault.
  • 6
    Chafed wiring causing intermittent open-circuit condition.
  • 7
    Incorrect replacement heater with mismatched resistance specification.

Symptoms drivers notice

MIL illuminated.
DEF system heater fault warning on the instrument cluster (cold-weather vehicles).
DEF freezing or slow thawing in sub-zero temperatures leading to dosing delays.
Secondary SCR or DEF pressure faults in very cold conditions.
No perceptible drivability symptoms in warm weather.

How to diagnose P25CB

A typical diagnostic flow when this code is present.

  1. 1
    Connect a scan tool, record all DTCs, and note ambient temperature at fault set.
  2. 2
    Inspect the Bank 1 Heater 2 connector and wiring harness for damage, corrosion, or loose terminals.
  3. 3
    Measure circuit resistance across the heater element terminals and compare to specification.
  4. 4
    Check for voltage at the heater control wire with the heater commanded on via scan tool actuator test.
  5. 5
    Inspect the heater relay or driver circuit for correct switching operation.
  6. 6
    Clear DTCs and monitor the heater circuit live data during a cold-start cycle.
  7. 7
    Replace the heater element or wiring harness segment if resistance is out of specification.

Vehicles where we've handled P25CB

Platforms in our catalogue with confirmed P25CB coverage.

BMW 320D
2016

Related powertrain codes

Frequently asked questions

Will P25CB cause a de-rate in summer?

Unlikely in warm weather, as DEF freezing is not a concern. However, the MIL will remain on and the fault should be repaired before cold weather returns.

Where is Bank 1 Heater 2 located?

Heater 2 typically refers to the second heating element in the DEF circuit, often located in the DEF line or pump assembly. Consult the vehicle-specific wiring diagram for the exact location.

Can I clear P25CB without repair?

The DTC will return if the underlying heater circuit fault is still present. The root cause must be corrected for a lasting repair.

Is this fault covered under emissions warranty?

DEF system components are often covered under the federally mandated emissions warranty. Check the vehicle warranty documentation for coverage details.

Disabling P25CB in software

RaceTune can permanently disable P25CB — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.

Permanent
The monitor is disabled in the ECU itself — not just cleared. It cannot return.
Tailored to your file
Each patch is matched to your specific software version — never a one-size-fits-all file.
Reversible
The original file is always preserved. Reflash the stock to return the ECU to factory state.

ECUs with a P25CB disable in our catalogue

Confirmed coverage from our recipe database — we support many more families. Upload your file and our identifier will match it automatically.

  • Bosch EDC17C50 verified 1 software version

Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.

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