P2123

Accelerator Pedal Position (APP) Sensor 1 Circuit High Voltage

P2123 is a generic OBD-II powertrain diagnostic trouble code: Accelerator Pedal Position (APP) Sensor 1 Circuit High Voltage. It is logged by the engine control unit when the scr/adblue monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.

Code
P2123
Group
Powertrain
System
SCR/AdBlue
Severity
Critical (limp mode / no-start)
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What P2123 means

P2123 is a generic OBD-II powertrain code defined as "Throttle/Pedal Position Sensor/Switch 'D' Circuit High Input." It is exclusive to drive-by-wire (electronic throttle control) systems, where there is no mechanical cable between the accelerator pedal and the throttle body. The PCM monitors multiple redundant sensor circuits — designators A through F — to cross-validate accelerator pedal position and throttle plate angle. Circuit D is typically the second accelerator pedal position (APP) sensor signal on platforms that use D/E pairing for pedal redundancy, or a third TPS channel on others.

When the PCM detects that the voltage on the D circuit exceeds its calibrated upper threshold (often above ~4.7 V), it cannot trust the reported pedal angle and flags P2123. This most commonly results from a short to voltage in the sensor signal wire, an internal sensor failure with the wiper stuck high, or a damaged connector allowing a supply-rail voltage to bleed onto the signal pin. Excessive carbon deposits holding the throttle plate partially open can also produce a misleading high-voltage condition during the throttle relearn sequence.

Because the PCM can no longer confirm driver intent, it immediately imposes a forced-idle or severely limited throttle strategy (limp mode) to prevent unintended acceleration. The MIL is illuminated on the first qualifying drive cycle. Normal operation is not restored until the fault is repaired and a throttle relearn procedure is completed with a scan tool.

Common causes

Most-frequently reported root causes when P2123 is logged.

  • 1
    Short circuit to voltage (B+) on the TPS/APP D-circuit signal wire.
  • 2
    Faulty accelerator pedal position sensor with internal wiper stuck at the high end of travel.
  • 3
    Corroded, spread, or water-contaminated connector pins at the pedal assembly or ECU harness.
  • 4
    Damaged wiring insulation causing the signal wire to contact a power supply wire.
  • 5
    Throttle body TPS-D element failure (on platforms where D designates a TPS rather than APP circuit).
  • 6
    Excessive carbon buildup on the throttle plate causing a false high reading during throttle relearn.
  • 7
    PCM/ECU internal fault misinterpreting the D-circuit voltage (rare, diagnose last).

Symptoms drivers notice

Malfunction Indicator Lamp (MIL / Check Engine Light) illuminated.
Engine enters limp mode — throttle response locked to a low fixed percentage or forced idle.
Severe loss of acceleration; vehicle may be undriveable above idle speed.
Possible engine stall, particularly at low-speed manoeuvres.
Throttle relearn procedure fails or cannot be initiated until the fault is cleared.

How to diagnose P2123

A typical diagnostic flow when this code is present.

  1. 1
    Connect a scan tool, confirm P2123 is current, and record freeze-frame data (RPM, throttle %, pedal %).
  2. 2
    Perform a visual inspection of the accelerator pedal assembly connector and harness for chafing, corrosion, or spread terminals.
  3. 3
    With KOEO (key on, engine off), measure D-circuit signal voltage at the PCM connector; if above ~4.7 V with pedal released, suspect wiring short to power.
  4. 4
    Wiggle-test the pedal harness while monitoring live D-circuit voltage on a scan tool to identify intermittent shorts.
  5. 5
    Unplug the pedal assembly connector; measure signal-pin voltage in the harness side — if voltage remains high, the fault is in the wiring or PCM, not the sensor.
  6. 6
    If wiring checks pass, substitute the accelerator pedal assembly with a known-good unit and retest.
  7. 7
    After repair, clear codes, perform the manufacturer throttle relearn procedure, and verify no recurrence over a complete drive cycle.

Vehicles where we've handled P2123

Platforms in our catalogue with confirmed P2123 coverage.

BMW 320D
2016

Related powertrain codes

Frequently asked questions

Can I still drive with P2123 stored?

The vehicle will likely be in limp mode with severely restricted throttle response. While the car may technically move, driving it is not recommended — acceleration will be minimal and the condition could worsen if an intermittent short becomes permanent.

Why does my car need a throttle relearn after fixing P2123?

Drive-by-wire systems store a baseline throttle-plate position in the PCM. After any repair touching the throttle body or pedal sensor, the PCM must relearn the closed-throttle voltage and idle airflow; without this the idle quality and throttle response will remain incorrect.

Could P2123 be caused by a bad PCM?

Yes, but it is rare. A PCM fault should only be suspected after all wiring, connectors, and the pedal sensor itself have been tested or replaced and the fault persists.

Does P2123 always mean the pedal sensor needs replacing?

Not necessarily. A short to voltage in the signal wiring — without any sensor damage — is a common root cause. Always test the wiring and connector before condemning the sensor, as harness repairs are significantly cheaper.

Disabling P2123 in software

RaceTune can permanently disable P2123 — 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 P2123 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

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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