P2122 and P2123: accelerator pedal position sensor D circuit

There is no cable from the pedal to the engine. The pedal contains sensors, the ECU reads them, and a motor moves the throttle accordingly.

P2122 and P2123 say those pedal sensors are producing an implausible signal.

Code	Meaning
P2120	Pedal position sensor D circuit
P2122	Circuit low input
P2123	Circuit high input
P2127, P2128	Sensor E equivalents

Related: P0121 to P0123 throttle position sensor.

Pedal, not throttle

The most useful thing this code does is tell you where to look.

A pedal sensor code means the throttle body is not the problem. Cleaning it, replacing it, or relearning its adaptation will change nothing.

The pedal assembly is inside the cabin, dry, accessible, and usually replaceable as a unit in under an hour. That is a much better repair than an engine bay component.

Why there are two sensors in one pedal

Redundancy, deliberately designed so that a single failure is always detectable.

The two sensors have different voltage slopes — one might rise from 0.5 V to 4.5 V across the travel while the other rises from 0.25 V to 2.25 V. That asymmetry is intentional: if the two wires short together, both read the same value, and the ECU knows immediately that something is wrong.

P2138 covers that disagreement specifically: P2135 and P2138 pedal sensor correlation.

Symptoms

• Sudden loss of power, engine limited to idle or a low ceiling.
• No response to the pedal, or response only over part of its travel.
• A dead spot where the engine does not react.
• Erratic acceleration without pedal movement.
• Cruise control unavailable.

The response is limp mode by design: limp mode, what it is and how to exit.

Causes

Wear in the sensor track

The pedal is used thousands of times per journey, always over the same first third of its travel. That is where potentiometric tracks wear.

Symptom: a dropout at a specific pedal angle, particularly light throttle in traffic.

The connector

Under the dash, in the footwell — reachable by feet, cleaning water, and anything spilled.

Water in the footwell

Worth its own mention. A blocked scuttle drain, a failed door seal or a leaking heater matrix puts water where this connector lives.

If the carpet on the driver’s side is damp, that is likely the whole story: damp carpet and electrical faults and water leak into cabin.

Floor mats

Mundane and genuinely dangerous. A mat that has slipped can hold the pedal partly depressed or block its return. Check before diagnosing electronics.

Aftermarket installations

Anything wired under the dash — alarms, remote starters, pedal “performance” boxes — can interfere. Pedal signal boosters in particular sit directly in this circuit: aftermarket accessories electrical faults.

The test

Log both pedal sensor values together, then press the pedal slowly from rest to fully down and back.

Observation	Meaning
Both rise smoothly, different slopes	Healthy
One drops out momentarily	Worn track at that angle
One stays at a fixed value	Open or shorted circuit
Both identical	Shorted together — see P2138
Both jump when the harness is moved	Connector or wiring

Press slowly. A quick press passes over a worn spot too fast to record.

After replacement

Some vehicles require a pedal adaptation reset after the assembly is replaced, so the ECU learns the new rest position.

Skipping it produces symptoms that look like a faulty new part: post-repair verification OBD checklist.

Where to start

1. Check the floor mat and the pedal’s free movement.
2. Check the carpet for damp.
3. Log both sensor values through a slow full press.
4. Inspect the connector under the dash.
5. Check for aftermarket wiring.
6. Replace the pedal assembly if a track has a dropout.
7. Perform any required adaptation afterwards.

Try it on the free tier of the app — pair an ELM327 adapter and record both pedal sensors through a slow full press. A dropout on that graph is the whole diagnosis, and it points at a part inside the cabin.