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.

What does GM code 21 mean?

21 — Throttle Position Sensor Error Signal High. This code sets when your vehicle computer detects 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

1. Read TPS voltage through the ALDL port at closed and wide open throttle
2. Check for binding cables or carbon buildup holding the throttle open
3. Backprobe the signal wire and sweep the throttle for smooth voltage change
4. Inspect the reference and ground circuits before replacing the sensor
5. Adjust or replace the sensor only if voltage stays above specification