P25C8
Reductant Heater Control Circuit High - Heater AP25C8 is a generic OBD-II powertrain diagnostic trouble code: Reductant Heater Control Circuit High - Heater A. 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.
What P25C8 means
P25C8 is set when the SCR control module detects a higher-than-expected voltage or an open-load condition on the control circuit for reductant heater A. AdBlue heating elements are critical in cold climates to prevent DEF from freezing below approximately -11 degrees Celsius. The module monitors the heater circuit during commanded operation and flags this code when the circuit voltage remains high while the heater is commanded off, or the measured current is outside the expected window.
Heater A typically refers to the first heater element in the reductant supply system, which may be located in the DEF tank, supply line, or pump module depending on the vehicle platform. A circuit-high condition usually points to an open circuit (broken wire or failed relay) that causes the module to sense full supply voltage on the feedback line rather than a loaded voltage.
Begin diagnosis by reading all active and pending DTCs to establish context, then perform a visual inspection of the heater circuit wiring, relay, and fuse. Compare live data heater current readings against specifications before condemning any component.
Common causes
Most-frequently reported root causes when P25C8 is logged.
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1
Open circuit in the heater A control or ground wiring.
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2
Failed heater relay stuck open or with an open control coil.
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3
Blown fuse in the reductant heater A power supply circuit.
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4
Corroded connector at the DEF tank heater or inline harness connector.
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5
Failed reductant heater element with an internal open.
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6
Faulty SCR or AdBlue control module output driver for heater A.
Symptoms drivers notice
How to diagnose P25C8
A typical diagnostic flow when this code is present.
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1
Retrieve all DTCs and note any accompanying reductant temperature or system-pressure faults.
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2
Check the heater A fuse and relay for continuity and proper operation.
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3
Inspect the heater A wiring harness and connector for open circuits, corrosion, or damage.
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4
Measure resistance of the heater element itself and compare to the manufacturer specification.
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5
Command the heater on with a scan tool and measure the circuit voltage at the heater connector.
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6
If the heater element and wiring are good, test the control module output for correct voltage switching.
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7
Replace the failed component (fuse, relay, heater, or module) and verify the repair by clearing codes and re-testing.
Vehicles where we've handled P25C8
Platforms in our catalogue with confirmed P25C8 coverage.
Related powertrain codes
Frequently asked questions
Is P25C8 only relevant in winter?
The heater circuit is commanded any time ambient or fluid temperature is below the threshold, but the code can set year-round if the circuit has an open fault that the module detects during self-test.
What is 'Heater A' specifically?
The designation A refers to the first heater in a system that may have multiple heater elements; exact location varies by manufacturer but commonly refers to the tank or supply-line heater.
Can a blown fuse cause P25C8?
Yes. A blown fuse creates an open circuit, which is a typical cause of a circuit-high code because the module sees unswitched voltage on the sense line.
Will the SCR system still dose AdBlue with this code active?
Dosing may continue if fluid temperature is above the freeze threshold, but the system cannot protect the fluid in cold conditions and may suspend operation if fluid temperature drops too low.
Disabling P25C8 in software
RaceTune can permanently disable P25C8 — 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.
ECUs with a P25C8 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
- Bosch EDC17CP09 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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