P26EC
Reductant Heater Control Circuit High (Bank 1)P26EC is a generic OBD-II powertrain diagnostic trouble code: Reductant Heater Control Circuit High (Bank 1). 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 P26EC means
P26EC is set when the SCR control module detects a higher-than-expected voltage on the control circuit for the reductant heater on Bank 1. The reductant heater warms the DEF fluid and supply lines in cold weather to prevent freezing and ensure the pump can deliver fluid. The module monitors the heater driver circuit feedback and sets this fault when it detects a sustained high-voltage anomaly.
A circuit-high condition on the heater control line typically indicates a short to battery voltage somewhere in the wiring between the control module and the heater element or relay. It can also result from an open low-side driver (the circuit floats high if the pull-up is to voltage and the ground path is open) or an internal fault in the SCR module heater driver stage.
Because this fault affects cold-weather operation, vehicles in warm climates may not exhibit any immediate driveability complaint. However, in freezing temperatures the heater's failure to operate correctly can prevent DEF from flowing, leading to SCR system shutdown and elevated NOx emissions. Repair before the cold season is recommended.
Common causes
Most-frequently reported root causes when P26EC is logged.
-
1
Short to battery voltage in the heater control or relay drive wire.
-
2
Failed heater relay with contacts welded closed creating a back-feed condition.
-
3
Open ground path in the heater control circuit causing the sense line to float high.
-
4
Internal SCR module heater driver circuit failure.
-
5
Chafed wiring contacting a voltage source in the engine bay.
-
6
Incorrect heater element with different resistance characteristics.
-
7
Corroded connector forcing an unintended voltage path.
Symptoms drivers notice
How to diagnose P26EC
A typical diagnostic flow when this code is present.
-
1
Record all DTCs and freeze-frame data; note ambient temperature at time of fault.
-
2
Inspect the heater wiring harness and connector for visible damage or shorts.
-
3
With the key on and heater de-commanded, measure voltage on the heater control circuit; voltage present here indicates a short to power.
-
4
Check the heater relay for signs of welded contacts by measuring voltage at the heater element with the relay removed.
-
5
Measure heater element resistance and compare to specification to rule out an internal short.
-
6
Inspect the SCR module heater driver output with the harness disconnected; if voltage is still present on the module output pin, suspect an internal module fault.
-
7
Repair wiring or replace the relay or module as indicated, then retest.
Vehicles where we've handled P26EC
Platforms in our catalogue with confirmed P26EC coverage.
Related powertrain codes
Frequently asked questions
What does Bank 1 mean for a reductant heater?
On some SCR systems, multiple heater circuits warm different sections of the reductant system (tank, lines, injector). Bank 1 refers to the first or primary heater zone as defined by the manufacturer.
Is P26EC dangerous to drive with?
In warm weather the immediate risk is low; the heater is not needed. In cold weather the risk increases because DEF can freeze, halting dosing and triggering further SCR faults that may result in a derate.
Can this code be caused by a blown fuse?
A blown heater fuse would remove power from the heater circuit and more likely set a circuit-low or open code. A circuit-high code suggests excess voltage, pointing toward a short to power rather than an open circuit.
Does the vehicle still heat the DEF when this code is set?
Not reliably. The fault indicates the heater control circuit is not behaving as commanded, so actual heater operation is uncertain. The SCR module may disable the heater driver to prevent further damage.
Disabling P26EC in software
RaceTune can permanently disable P26EC — 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 P26EC 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 EDC17C56 verified 1 software version
- Bosch MD1CP002 verified 1 software version
- Bosch MD1CS001 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 P26EC 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