Is the 'D' sensor in the throttle body or the accelerator pedal?

On the overwhelming majority of vehicles it is in the accelerator pedal assembly, despite the word 'Throttle' appearing first in the SAE title. The throttle body's own position sensors have separate codes at P0120 through P0124 and in the P0220 range. Because a small number of manufacturers assign the letters differently, the safe move is to confirm in your service data which sensor is designated 'D' on your platform before ordering anything. Buying a throttle body for a pedal sensor fault is an expensive way to learn this.

How does P2120 differ from P2121, P2122 and P2123?

P2120 is the unqualified parent of the other three. P2121 means the signal is present but implausible for the conditions. P2122 means it is stuck at or near zero volts. P2123 means it is stuck at or near reference voltage. Because P2120 does not tell you the direction of the fault, it gives no shortcut — you have to check reference voltage, ground, signal continuity and the sensor's actual sweep. Some manufacturers only ever set P2120 and never use the specific three, in which case it simply covers the whole family on that vehicle.

Why is my engine in reduced power mode?

Because the module no longer trusts what the pedal is telling it, and the safe response to an untrustworthy accelerator signal is to limit engine output rather than risk acting on a wrong reading. This is a deliberate protective strategy, not evidence that the engine or transmission is damaged. On most vehicles the second pedal sensor is still working, which is what keeps the engine running and lets you creep along at limited power. Repair the 'D' circuit fault and normal throttle response returns immediately.

Can I keep driving with P2120?

You can move the car, but you should not rely on it. Power is capped, acceleration is poor, and merging onto a highway or pulling across traffic in a vehicle that will not accelerate is a real hazard. There is also the possibility that a partially broken wire becomes a fully broken one and the second sensor faults as well, at which point the module can force idle or shut the engine down. Braking and steering are unaffected, so getting it home or to a shop at low speed on quiet roads is reasonable — treating it as a normal daily driver is not.

OBD-II trouble code

P2120: Throttle/Pedal Position Sensor 'D' Circuit

The general circuit fault for the 'D' sensor inside the accelerator pedal assembly. It is the unqualified parent of P2121, P2122 and P2123 — it says the signal is unacceptable without saying whether it is too high, too low, or simply implausible.

High severityPowertrainThrottle / IdleDrivable short-term

Quick facts

System: Powertrain
Category: Throttle / Idle
Severity: High severity
Drivable: Usually safe to drive short-term
Repair cost range: $0 – $550
DIY difficulty: Intermediate DIY

What does P2120 mean?

On a drive-by-wire vehicle nothing mechanical connects the accelerator pedal to the throttle. The pedal assembly contains two independent position sensors — labeled 'D' and 'E' on many platforms, 'A' and 'B' on others — and the engine control module reads both. Two sensors exist so the module can cross-check them. If they agree, the module trusts the reading and opens the throttle to match. If either one produces a signal the module cannot accept, it stops trusting the driver's pedal entirely and falls back to a reduced-power strategy, because the alternative — acting on a pedal signal that might be wrong — is an unintended acceleration risk.

P2120 is the broad, unqualified member of the 'D' sensor family. Its three siblings each tell you something specific: P2121 says the signal is present but implausible for the conditions, P2122 says it is stuck at or near zero volts, and P2123 says it is stuck at or near reference voltage. P2120 says only that the circuit is faulted. That lack of qualification is itself the diagnostic starting point — you cannot assume a direction, so you check the reference voltage, the ground, the signal wire and the sensor's actual sweep rather than jumping to a conclusion. Some manufacturers set P2120 as their only 'D' circuit code and never use the more specific three, in which case it covers the whole family on that platform.

The most useful thing to be clear about is what part this code points at. Despite the word 'Throttle' appearing in the SAE title, on the overwhelming majority of vehicles the 'D' sensor lives in the accelerator pedal assembly, not the throttle body. Throttle body sensor faults have their own codes at P0120 through P0124 and P0220 onward. Chasing a P2120 with a throttle body is a common and avoidable mistake — check your service data to confirm which sensor your manufacturer designates 'D' before buying anything.

Because the pedal assembly sits under the dash rather than in the engine bay, the failure pattern is different from most sensor codes. There is no heat cycling, no road salt, and no oil contamination. What there is instead is mechanical abuse: wiring crushed by items stored under the front seat, connectors kicked loose by feet, harnesses disturbed during carpet or trim work, and the occasional spilled drink finding its way down to the connector. Internal sensor failure and pushed-back connector pins account for most of the rest.

Common causes

Open, shorted, or chafed wiring in the accelerator pedal harness
Wiring crushed or pinched by items stored under the front seat
Pushed-back pin, damaged locking tab, or corrosion at the pedal connector
Internal failure of the 'D' position sensor inside the pedal assembly
Open or shorted 5-volt reference circuit to the pedal sensor
Open or high-resistance sensor ground circuit
Liquid spill reaching the pedal connector or the assembly itself
Harness disturbed during recent carpet, trim, or under-dash work
Aftermarket alarm, remote start, or pedal-mounted accessory wiring spliced into the circuit
Failed pedal assembly — the sensors are integrated and not serviceable separately
Failed engine control module input circuit (rare — confirm wiring and sensor first)

Symptoms

Check engine light on, frequently at key-on before the vehicle is even driven
Reduced engine power warning on the dash
Limp mode — throttle response severely limited regardless of pedal input
Engine starts and idles normally but does not respond fully to the pedal
Hesitation, surging, or inconsistent response when accelerating
Vehicle may be unable to reach highway speed
Symptom sometimes clears at the next key cycle and returns while driving
Companion codes such as P2138 or a fail-safe code stored alongside

Diagnostic steps

1.Scan every module and record all stored and pending codes with freeze frame data. P2120 alone points at the 'D' circuit; P2120 together with an 'E' sensor code points at shared wiring, reference, or ground rather than at either sensor individually.
2.Confirm in service data which physical sensor your manufacturer designates 'D'. On most vehicles it is a sensor inside the accelerator pedal assembly, not the throttle body — verify before buying any part.
3.Test the battery and charging system. Low system voltage distorts sensor reference voltages and can set pedal circuit codes on a healthy sensor.
4.Locate the pedal assembly under the dash and inspect the harness routing for crush damage, pinch points, and evidence of items stored under the seat.
5.Inspect the pedal connector for backed-out pins, a properly seated locking tab, corrosion, and any sign of liquid intrusion.
6.Disconnect the pedal connector. With the key on, verify approximately 5 volts on the reference pin and near-zero volts on the sensor ground pin, measured against a known-good chassis ground.
7.Check the signal wire for continuity to the module and for shorts to power and to ground with both ends disconnected.
8.Reconnect the pedal and watch the 'D' sensor value in live data while slowly pressing the pedal through its entire travel. The signal should sweep smoothly from its rest value to its full-press value with no flat spots, dropouts, or sudden jumps.
9.Display the 'D' and 'E' sensor signals together. On most platforms they track each other proportionally; a divergence identifies which of the two is at fault.
10.Perform a wiggle test on the pedal harness with live data displayed, flexing the harness and the connector while an assistant watches for a momentary dropout.
11.If reference, ground and signal wiring all test good but the sensor value is wrong or erratic when connected, replace the pedal assembly and perform the pedal relearn procedure for the platform.

Repair cost

$0 – $550

The low end is zero because a connector that only needs to be reseated, or a pin pushed back into place, is a genuine no-parts fix — and this is a real outcome on a connector that lives where feet and cargo can reach it. Wiring repair typically runs $100-$300 with diagnostic time. An accelerator pedal assembly is the usual part when the sensor itself has failed, at roughly $150-$400 installed on mainstream vehicles and up to $500 or more on luxury platforms or where the pedal is integrated into a larger assembly requiring more disassembly. The pedal itself is usually held by two or three nuts, so labor is modest once the diagnosis is confirmed. Many platforms require a pedal relearn procedure afterward.

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DIY vs shop

This is an intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.