P255E

Speed Selector Sensor/Switch 2 Circuit Intermittent/Erratic

P255E is a generic OBD-II powertrain diagnostic trouble code: Speed Selector Sensor/Switch 2 Circuit Intermittent/Erratic. 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
P255E
Group
Powertrain
System
Powertrain
Severity
Warning (MIL on)
Need P255E disabled?
RaceTune permanently disables any OBD-II trouble code on supported ECUs — for motorsport, off-road, and export use.
→

What P255E means

P255E is stored when the ECM detects an electrical fault in the heater control circuit for the downstream NOx sensor (Bank 1 Sensor 2). The heater is essential for bringing the NOx sensor's ceramic sensing element up to its operating temperature quickly and maintaining it there during cold or low-load operation. Without a functioning heater, the sensor may produce inaccurate readings or no signal at all, undermining SCR feedback control.

The ECM drives the heater through a dedicated driver stage and monitors the resulting current or voltage feedback. An open circuit (broken heater element, disconnected connector, or broken wire) or a short circuit (internal insulation failure in the heater element) will cause the monitored value to fall outside expected bounds, triggering this code. A failed ECM heater driver can also set P255E though this is less common.

Cold-climate operation and extended idling, where condensation can enter harness connectors, accelerate connector corrosion and are a frequent context in which P255E first appears. Resolving heater circuit faults promptly prevents secondary NOx sensor signal faults from developing.

Common causes

Most-frequently reported root causes when P255E is logged.

  • 1
    Open circuit in the NOx sensor heater element (internal break in the resistive element).
  • 2
    Short circuit within the heater element causing overcurrent protection to activate.
  • 3
    Broken or corroded heater supply or ground wire in the harness.
  • 4
    Water ingress into the sensor connector corroding heater circuit terminals.
  • 5
    Failed ECM heater driver output for the downstream NOx sensor channel.
  • 6
    Incorrect replacement sensor with incompatible heater resistance.
  • 7
    Wiring harness damage from heat or mechanical abrasion near the exhaust.

Symptoms drivers notice

MIL illuminated.
Downstream NOx sensor slow to reach operating temperature, especially during cold starts.
Potential secondary NOx performance or range codes if the cold sensor produces implausible readings.
SCR system may operate in open-loop mode until sensor reaches temperature.
No significant engine driveability change in most cases.

How to diagnose P255E

A typical diagnostic flow when this code is present.

  1. 1
    Connect a scan tool, record all DTCs and note any companion signal or performance codes for the same sensor.
  2. 2
    With ignition off, disconnect the NOx sensor and measure heater element resistance at the sensor connector; compare with manufacturer specification.
  3. 3
    Check heater supply voltage at the connector with ignition on and engine off.
  4. 4
    Measure continuity of the heater ground wire back to chassis ground.
  5. 5
    Inspect the connector and wiring for corrosion, moisture damage, or chafing.
  6. 6
    If wiring and heater resistance are out of specification, replace the downstream NOx sensor.
  7. 7
    Clear codes and perform a cold-start drive cycle to confirm the heater circuit operates correctly.

Vehicles where we've handled P255E

Platforms in our catalogue with confirmed P255E coverage.

BMW 320D
2016

Related powertrain codes

Frequently asked questions

Can P255E cause a failed emissions test?

Yes, because the MIL is on and SCR monitoring may be compromised, leading to an automatic test failure in most jurisdictions.

What is a typical heater resistance range for a NOx sensor?

Heater resistance varies by design; always consult the specific vehicle service data rather than assuming a generic value.

Can moisture cause P255E?

Yes. Water ingress into the connector is a common cause of intermittent heater circuit faults, particularly in vehicles operated in wet or salty conditions.

Will replacing just the sensor fix P255E?

If the wiring and ECM driver are healthy and only the heater element is faulty, yes. Always verify wiring integrity first to avoid unnecessary sensor replacement.

Disabling P255E in software

RaceTune can permanently disable P255E — 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 P255E 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 P255E 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