How to test throttle and accelerator pedal position sensors (dual track)
With drive-by-wire, the ECU reads the pedal position, opens the throttle with a motor and checks the throttle position sensors against what it commanded. A worn track, a bad connector or a dirty throttle body causes hesitation, a high or hunting idle and limp mode with reduced power. After cleaning or replacing a throttle body, most engines need an adaptation (throttle relearn).
Updated · written by ipacen; typical values, not manufacturer data
Electronic throttles and accelerator pedals use two position signals for safety, each with a 5 V reference and a ground. With the ignition on, sweep slowly: track 1 typically rises from about 0.5–1 V at rest to about 4–4.5 V at full travel, and track 2 either mirrors it the other way or follows it at a different level (often about half). Both must change smoothly with no dropouts and keep the same relationship through the whole travel; a mismatch sets a correlation code and limp mode.
Typical values
| Check | Typical good | Suspect |
|---|---|---|
| 5 V reference | About 4.9–5.1 V | Low or missing (often shared with other sensors) |
| Track 1 at rest / full travel | About 0.5–1 V / 4–4.5 V (check the spec) | Outside the range |
| Track 2 | Mirrors or follows track 1 as specified | Wrong relationship anywhere in the travel |
| Slow sweep on a scope | Smooth, no steps or dropouts | Dropouts or noise at one position |
| Throttle opening at warm idle | Small and steady (a few %) | High: dirty throttle or an air leak |
Typical ranges, not manufacturer data: check the specification for the engine.
Wiring: what you should measure
Pedal or throttle position sensor (two tracks)
| Wire | What you should see |
|---|---|
| 5 V reference (two on many pedals) | About 4.9–5.1 V with the ignition on |
| Ground (two on many pedals) | Less than about 0.1 V drop |
| Signal 1 | About 0.5–1 V at rest, rising to about 4–4.5 V at full travel |
| Signal 2 | A second track: mirrored, or at a different level (often about half of signal 1) |
Throttle motor (electronic throttle body)
| Wire | What you should see |
|---|---|
| Motor + and − | Driven by the ECU with a pulsed signal in both directions; never power it directly with the connector on |
Before you start
- Read the codes: P0120/P0220 families point to a sensor circuit, P2135/P2138 to the two tracks disagreeing, P2101/P2111/P2112 to the throttle motor.
- Inspect the connectors at the pedal and the throttle body for corrosion and loose pins, and the throttle body for carbon.
Scan tool
- Watch both pedal tracks and both throttle tracks in live data while pressing the pedal slowly.
- Compare commanded and actual throttle angle.
- After cleaning or replacing a throttle body, run the throttle adaptation with the conditions the procedure asks for.
Multimeter
- Measure the 5 V reference and the ground at the sensor connector.
- Back-probe each signal and sweep slowly from rest to full travel.
Oscilloscope
- Both tracks on two channels, about 1 V/div and 500 ms/div, key on and engine off.
- Sweep slowly up and down: look for dropouts or steps, and check the two tracks stay in step.
What it looks like on a scope
Typical shapes drawn by ipacen, not a capture from one car: amplitude and timing vary with the engine, the sensor and the rpm.
What the readings tell you
| You find | It usually means |
|---|---|
| Both tracks wrong together | 5 V reference or ground fault (often shared with other sensors) |
| Dropouts at one position | Worn sensor track: replace the pedal or the throttle body |
| Correlation code with good wiring | Sensor fault; check the connector pins for corrosion first |
| High idle after cleaning the throttle | Adaptation not done: run the throttle relearn |
| Actual throttle lags commanded | Sticking throttle (carbon) or a weak throttle motor |
Known engine problems behind these codes
Questions mechanics ask
Do I need to relearn the throttle body after cleaning it?
On most electronic throttles, yes. The ECU has learned the old closed position with carbon in it; without an adaptation the idle can be high or unstable. Run the throttle relearn with a scan tool or the procedure for the car.
What does P2135 mean?
The two throttle position tracks disagree. Check the 5 V reference, the ground and the connector first; if they are fine, the sensor (usually part of the throttle body) is faulty.
Can I test the throttle motor with 12 V?
Only briefly on the bench, with the connector off, to see whether it moves freely. Never apply voltage with the ECU connected. In the car, use the throttle actuator test and compare commanded and actual angle.
Values are typical, not manufacturer data. Always follow the service information for the vehicle and work safely.