P25E2
Reductant Heater Control Circuit - OpenP25E2 is a generic OBD-II powertrain diagnostic trouble code: Reductant Heater Control Circuit - Open. 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 P25E2 means
P25E2 is stored when the control module detects an open circuit in the reductant heater control circuit. The DEF heating system keeps reductant fluid from freezing at temperatures below approximately -11 degrees Celsius. The heater may be located in the DEF tank, supply lines, or dosing module, and is typically commanded by the control module through a relay or direct driver circuit.
An open circuit prevents current from flowing through the heater element. In cold climates this means the DEF fluid can freeze, blocking delivery and causing companion DEF flow or dosing faults. In warm climates the open may go unnoticed for flow purposes but the fault will still set and illuminate the MIL because the circuit is monitored continuously.
Diagnosis should focus on the heater element resistance, associated relay, fuse, and the wiring between the control module and heater. Corrosion at connector pins in the DEF module is a frequent cause due to the aggressive chemistry of DEF fluid when leaked.
Common causes
Most-frequently reported root causes when P25E2 is logged.
-
1
Open or broken wire in the reductant heater supply or control circuit.
-
2
Blown fuse protecting the reductant heater circuit.
-
3
Failed heater element with an open coil (infinite resistance).
-
4
Failed heater relay not passing current to the element.
-
5
Corroded or disconnected heater connector due to DEF fluid exposure.
-
6
Faulty control module output driver for the heater circuit.
Symptoms drivers notice
How to diagnose P25E2
A typical diagnostic flow when this code is present.
-
1
Retrieve all DTCs and freeze-frame data with a scan tool.
-
2
Check the fuse protecting the reductant heater circuit and replace if blown.
-
3
Inspect the heater connector and wiring for corrosion, DEF contamination, or open conductors.
-
4
Measure heater element resistance at the connector; an open reading confirms element failure.
-
5
Verify the heater relay operation by applying voltage directly to the relay coil and checking continuity across the load contacts.
-
6
If wiring and components test good, check for loss of control signal from the module and inspect module connector for corrosion.
-
7
Repair or replace the faulty component and verify the circuit is functional with a scan tool live data check.
Vehicles where we've handled P25E2
Platforms in our catalogue with confirmed P25E2 coverage.
Related powertrain codes
Frequently asked questions
Will this code cause an engine derate?
P25E2 alone typically sets the MIL without an immediate derate, but if the heater failure leads to frozen DEF and dosing faults, derate countdowns can begin.
How do I test the heater element?
Unplug the heater connector and measure resistance across the heater terminals with a multimeter. An open-circuit reading (OL or infinite) confirms a failed element.
Can DEF fluid damage the heater wiring?
Yes. DEF is a urea solution that crystallizes when dried and is corrosive to some metals. Leaking DEF at connectors causes accelerated pin corrosion and open circuits.
Does the heater run all the time?
No. The control module activates the heater based on coolant temperature and ambient temperature signals. It is typically energized during cold starts and in cold ambient conditions until fluid temperature rises.
Disabling P25E2 in software
RaceTune can permanently disable P25E2 — 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 P25E2 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.
Got P25E2 in your scan?
Upload your ECU file — we'll identify the exact software version and confirm whether a disable is available for your car.
Upload your file