P255D
Speed Selector Sensor/Switch 2 Circuit HighP255D is a generic OBD-II powertrain diagnostic trouble code: Speed Selector Sensor/Switch 2 Circuit High. 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 P255D means
P255D is set when the ECM/PCM detects that the signal voltage from the downstream NOx sensor (Bank 1 Sensor 2, after the SCR catalyst) exceeds the maximum calibrated threshold. A persistently high voltage on the signal circuit points to a short to voltage somewhere in the wiring, an open return/ground path for the sensor, or an internally failed sensor that is driving its output rail-high.
Because the downstream NOx sensor provides the primary closed-loop feedback for SCR catalyst efficiency and DEF dosing, a rail-high signal prevents accurate emissions management. The ECM will typically disable closed-loop NOx control and revert to a fixed dosing strategy, which can result in either excessive DEF consumption or insufficient NOx conversion depending on the fallback calibration.
Wiring faults in the high-heat exhaust routing environment are a common contributor. Insulation breakdown in the harness can create a path from a 5 V or 12 V reference line to the low-level signal wire, raising its apparent voltage above the plausible range. Thorough wiring inspection is recommended before sensor replacement.
Common causes
Most-frequently reported root causes when P255D is logged.
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1
Short to voltage on the NOx sensor signal wire from an adjacent reference or supply wire.
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2
Open circuit in the sensor ground/return path causing the signal to float high.
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3
Internally failed NOx sensor driving signal output to maximum voltage.
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4
Wiring insulation breakdown due to heat exposure near the exhaust tunnel.
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5
Incorrect sensor fitted with a mismatched output range.
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6
ECM internal fault on the NOx sensor input pull-up circuit.
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7
Corroded connector forcing signal wire to contact a higher-potential terminal.
Symptoms drivers notice
How to diagnose P255D
A typical diagnostic flow when this code is present.
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1
Retrieve all DTCs and freeze-frame data with a scan tool.
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2
Disconnect the NOx sensor and observe whether the signal voltage drops; a voltage that remains high points to a wiring fault rather than the sensor.
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3
Inspect the sensor harness for heat damage, chafing against hot surfaces, and contact with adjacent wiring.
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4
Measure the signal wire resistance to any voltage supply rail with sensor disconnected to identify a short to voltage.
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5
Check the sensor ground path continuity from connector to chassis ground.
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6
If wiring is intact, substitute a known-good downstream NOx sensor and retest.
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7
Clear codes and perform a complete drive cycle to verify the repair.
Vehicles where we've handled P255D
Platforms in our catalogue with confirmed P255D coverage.
Related powertrain codes
Frequently asked questions
Why does unplugging the sensor help diagnose P255D?
If the high voltage disappears when the sensor is unplugged, the sensor itself is the source. If it remains high, the fault is in the wiring or ECM.
Can P255D cause excessive DEF usage?
Yes. A rail-high signal may trick the ECM into thinking NOx levels are very high, triggering maximum DEF injection rates.
Is this code common after exhaust system repairs?
It can appear if the harness is disturbed and a wire is pinched or if the replacement sensor has damaged insulation.
Does P255D always mean sensor replacement?
No. A wiring short to voltage is a common root cause and must be ruled out before replacing the sensor.
Disabling P255D in software
RaceTune can permanently disable P255D — 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 P255D 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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