Throttle Position Sensor 1-2 Correlation Error (GM)
What does P1221 mean?
The P1221 code means the vehicle's onboard diagnostic system has detected: Throttle Position Sensor 1-2 Correlation Error (GM). This is a moderate severity code.
Common Symptoms
- Check Engine Light on
- Reduced Engine Power warning
- Sluggish acceleration
- Erratic throttle response
- Vehicle may limit speed to 25-30 MPH
Probable Causes (Ranked by Likelihood)
- Faulty throttle body with worn TPS sensors Very High
- Carbon buildup on throttle plate affecting position High
- Wiring issue causing signal discrepancy Moderate
- 5V reference voltage problem Moderate
- PCM calibration issue Low
Step-by-Step Diagnostic Procedure
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Compare TPS 1 and TPS 2 readings on a scan tool -- they should be inversely proportional
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Slowly sweep the throttle and watch for any point where the signals disagree
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Clean the throttle body to remove carbon buildup
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Check the 5V reference voltage at the throttle body connector
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Replace the throttle body if correlation error persists after cleaning
Frequently Asked Questions
What is a TPS correlation error?
The PCM expects TPS 1 and TPS 2 to produce complementary readings at all times. A correlation error means the two sensors are producing signals that do not match expected patterns, indicating a problem with the throttle body or wiring.
Can cleaning the throttle body fix P1221?
Sometimes. If carbon buildup is preventing the throttle plate from closing fully or moving smoothly, cleaning can restore proper TPS correlation. Try cleaning before replacing the throttle body.
Why is my vehicle limited to 25 MPH?
When the PCM detects a TPS correlation error, it activates Reduced Engine Power mode that severely limits throttle opening. This 25-30 MPH speed limit is a safety feature to prevent uncontrolled acceleration.