Faulty throttle body with a worn or failing internal throttle position sensor track; Dirty or carbon-fouled throttle body causing the throttle plate to stick or move unevenly; Damaged, chafed or corroded wiring or connector to the throttle body (signal, reference or ground circuits); Voltage reference or ground fault shared by the two throttle position sensor circuits; Poor connection or high resistance in the throttle body harness terminals; Fault in the engine control module or an outdated calibration, depending on model and engine.

P1221

Throttle Position (TP) Sensor 1 2 Correlation.

GMC
Powertrain
Manufacturer code
Medium severity

How this code is built

P: SystemPowertrain
1: Defined byThe manufacturer
2: AreaFuel and air metering (injector circuit)
21: FaultThe specific fault in that area

What it means

P1221 on a GMC means the engine control module has found that the two throttle position sensor signals inside the electronic throttle body do not agree with each other within the expected range. The two sensors should track each other closely at every throttle angle, so a mismatch suggests a fault in one sensor, its wiring or the throttle body. The module usually limits throttle response to protect the engine and driver.

Symptoms

Check engine light on, often with a reduced engine power warning
Limited throttle response or the engine held at a low power level
Hesitation or poor acceleration
Rough or unstable idle
Engine stalling or difficulty starting
Cruise control not working

Possible causes

Faulty throttle body with a worn or failing internal throttle position sensor tracklikely
Dirty or carbon-fouled throttle body causing the throttle plate to stick or move unevenlylikely
Damaged, chafed or corroded wiring or connector to the throttle body (signal, reference or ground circuits)possible
Voltage reference or ground fault shared by the two throttle position sensor circuitspossible
Poor connection or high resistance in the throttle body harness terminalspossible
Fault in the engine control module or an outdated calibration, depending on model and enginerare

How to diagnose

Read all stored codes and freeze frame data, noting engine speed, load and throttle position when the code set, and check for other throttle or accelerator pedal codes
Inspect the throttle body connector and harness for damage, corrosion, bent or spread terminals, and signs of rubbing or water ingress
Inspect the throttle body bore and plate for heavy carbon or deposits and check the plate moves freely by hand with the ignition off
With a scan tool, watch both throttle position sensor values from closed to wide open and back, checking that they track each other smoothly with no dropouts or jumps
Check the 5 volt reference and ground at the throttle body connector against the wiring diagram for your model and engine
Voltage drop test the signal, reference and ground circuits while wiggling the harness to find intermittent faults
Check for any technical service bulletins or calibration updates for your model and engine before replacing parts
Clear the codes and road test to see whether the fault returns and under what conditions

How to fix

Clean the throttle body using a method approved for your engine, then carry out any throttle relearn procedure required
Replace the throttle body assembly if the internal sensor readings do not correlate, then perform the relearn procedure
Repair or replace damaged wiring, connectors or terminals in the throttle body circuits
Repair the voltage reference or ground fault found in the circuit
Update or reflash the engine control module calibration only where an outdated calibration is a confirmed cause, or replace the module if it is proven faulty

Can I keep driving?

Medium severity

The car may be usable in a reduced power mode, but throttle response can be unpredictable and it may stall, so avoid long journeys and busy roads. Get it diagnosed soon.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.