One track of the accelerator pedal sensor is reading high. The pedal has two independent tracks precisely so the ECU can spot this.
P2123 — also written P2123, P 2123, 2123 — is the OBD-II code for throttle/pedal position sensor 'd' circuit high. Same code, same fix: this page covers every spelling of it.
P2123 is a engine fault code, typically costing £90–£400 to put right in a UK garage.
What causes P2123?
- Failed accelerator pedal sensor
- Water ingress into the pedal connector
- Chafed wiring in the footwell
- Spilt drink into the pedal assembly
How P2123 is fixed
- Compare both pedal tracks on live data
- Replace the accelerator pedal assembly
- Dry and repair the connector
P2123 repair cost in the UK
Most P2123 repairs land between £90–£400 including parts and labour. The spread is wide because the cheap fix (a sensor, a hose, a leak) and the expensive fix (a major component) throw the same code — which is why a proper diagnosis before parts are bought saves the most money.
Can you drive with P2123?
Limp mode. Do not drive far with an unreliable pedal signal.
Urgency: high.
What P2123 is telling you
The ECU runs continuous plausibility checks against what a good engine should be doing. When a reading strays outside that model for long enough, this code is stored and the light comes on.
On P2123 specifically — throttle/pedal position sensor 'd' circuit high — the stored code records the moment that check failed, not the moment the part gave up. That gap is why two cars with the same code can need completely different repairs.
How P2123 is diagnosed properly
Each of the known causes is ruled in or out in cost order, cheapest and most likely first. On P2123 that means working through them like this:
- Failed accelerator pedal sensor — compared against live data and a known-good reference before it is condemned — a sensor reading the wrong thing accurately is a different fault from a dead one.
- Water ingress into the pedal connector — opened, inspected for green corrosion and spread pins, and back-probed rather than unplugged, so the reading is taken with the circuit live.
- Chafed wiring in the footwell — checked with the loom flexed and the engine warm, because a wiring fault that is intermittent when cold shows itself under heat and vibration.
- Spilt drink into the pedal assembly — confirmed with live data before any part is bought.
An hour of diagnostic time on P2123 routinely saves more than it costs, because the alternative is buying the most expensive part on that list and hoping.
The repairs that actually clear P2123
- Compare both pedal tracks on live data
- Replace the accelerator pedal assembly
- Dry and repair the connector
After the repair the code is cleared and the car is driven until the relevant readiness monitor completes, rather than handed back as soon as the light goes out. A code that has only been erased will come straight back on the next drive cycle.
What happens if P2123 is ignored
Leaving this one is the expensive choice. The protection strategy the ECU falls into is there to limit damage, and every mile driven with the fault active is a mile the engine or gearbox spends outside its designed operating window.
For reference, P2123 typically lands at £90–£400 all-in at an independent garage in the UK — main-dealer quotes for the same job usually sit above that.
Getting P2123 looked at
Bob's Mechanical Repairs is an independent garage in Birnam, Dunkeld, Perthshire, with 32 years of Bob's experience on the tools. If P2123 is showing on your car, bring the scan results with you — we will read the live data ourselves, tell you which of the causes above it actually is, and price the repair before any work starts.