GM 21 Throttle Position Sensor Error Signal High
How to read it
Code 21 reads as 2 flashes, pause, 1 flash on the check-engine light. Jump terminals A and B on the ALDL connector with the key off, then turn the key on with the engine off and count the check-engine flashes.
New to flash codes? The How to Read GM OBD-I Codes with a Paperclip walks through the test and what the numbers mean. The general OBD-I guide covers the rest.
What does GM code 21 mean?
Throttle Position Sensor Error Signal High
Common causes of 21
- Throttle position sensor shorted to the 5 volt reference
- Sticking throttle linkage holding voltage high
- Damaged TPS wiring with signal shorted to power
- Misadjusted sensor reading above specification at closed throttle
How to fix 21
- Read TPS voltage through the ALDL port at closed and wide open throttle
- Check for binding cables or carbon buildup holding the throttle open
- Backprobe the signal wire and sweep the throttle for smooth voltage change
- Inspect the reference and ground circuits before replacing the sensor
- Adjust or replace the sensor only if voltage stays above specification
Sources and review
Review: qualified. Read the source limits below.
- GM and Chevrolet OBD1 Trouble Codes (ALDL Fault Code List)
live reference page (troublecodes.net)
Supports: code identity and GM meaning
Fetched directly. Single publisher, so the record is qualified, not verified. Causes and check order are curator-written from the circuit the code names.
Last updated 2026-09-19.
Report a correction for 21. Include your vehicle, the issue and a source if you have one.