Code
P1221
GM
P — Powertrain
Throttle Position (TP) Sensor 1 2 Correlation
AI status
Completed
Completed
100%
Causes
- Faulty throttle position sensor (one or both sensors)
- Damaged wiring, chafed harness, or poor connector/terminal contact
- Contaminated or sticking throttle body / throttle plate
- Poor reference voltage or ground to the sensors
- Faulty or misprogrammed powertrain control module (PCM/ECM)
Symptoms
- Check Engine MIL illuminated
- Reduced engine power or limp-home mode
- Poor idle, surging, or hesitation during acceleration
- Unresponsive or inconsistent throttle feel
- Possible trouble starting or stalling in some cases
What to check
- Read freeze frame and stored codes; note conditions when code set
- Scan live data: compare TP Sensor 1 and TP Sensor 2 values at key positions (closed, mid, WOT)
- Verify 5V reference supply and sensor ground at throttle body connector
- Inspect wiring and connector for corrosion, damage, or loose pins; wiggle test harness while monitoring data
- Visual inspection and cleaning of throttle body and throttle plate
- Perform continuity/resistance checks on sensor signal wires back to PCM if readings disagree
Signal parameters
- Typical closed-throttle voltage: ~0.4–0.6 V (sensor dependent)
- Typical wide-open-throttle voltage: ~4.2–4.6 V
- Both sensors should track together across the throttle sweep; difference should be small (usually
- Stable 5 V reference supply and low-resistance ground required for correct readings
Diagnostic algorithm
- Retrieve DTCs and freeze-frame data; note engine temp, rpm, throttle position when P1221 set.
- With a scan tool, monitor TP Sensor 1 and TP Sensor 2 values at closed throttle, during slow opening, and at WOT. Confirm they move smoothly and correlate.
- Inspect throttle body connector for corrosion, bent pins, or contamination. Repair/clean as needed and recheck.
- Verify 5 V reference and ground at the connector with a DVOM. If absent or noisy, trace wiring to PCM.
- Backprobe each sensor signal while operating throttle; look for intermittent drops, jumps, or one sensor not moving. Wiggle harness to reproduce fault.
- If wiring harness checks good, remove/clean throttle body; verify throttle plate moves freely. Re-scan after cleaning.
- If sensor signals remain out of correlation, replace the throttle body assembly or TPS (follow OEM guidance).
- After repair, clear codes and perform relearn/idle relearn or throttle adaptation procedure if required by the manufacturer. Road test and recheck for recurrence.
- If problem persists after replacement, consider PCM fault and perform further module-level diagnostics or reprogramming.
Likely causes
- Intermittent/loose connector at throttle body
- Short to ground or short to power on one sensor circuit
- Failed dual-element TPS inside electronic throttle body
- Carbon buildup causing throttle plate to stick and give inconsistent readings
- Aftermarket or incorrect replacement throttle body/sensor without calibration
Fault status
Status
ECM detected poor correlation between TP Sensor 1 and TP Sensor 2. Throttle control may be reduced and a diagnostic check of TPS circuits, wiring, and throttle body is recommended.
Repair difficulty: Medium
Diagnostic time: 1.0-2.5 hours
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