P1120
GMThrottle Position (TP) Sensor Circuit Intermittent High Voltage or Low Voltage
Medium SeverityThe Engine Control Module (ECM) has detected that the throttle position sensor (TPS) signal voltage is outside its expected operational range. This signal is crucial for the ECM to accurately determine the throttle plate angle, affecting engine performance and potentially leading to erratic idle or poor acceleration.
⚠️ Before You Begin (Safety First)
Always ensure the vehicle is parked on level ground with the parking brake engaged. Disconnect the negative battery terminal before working on electrical components to prevent accidental shorts or system damage.
Common Symptoms
- Illuminated Check Engine Light
- Hesitation or surge during acceleration
- Rough or erratic idle
- Engine stalls when coming to a stop
- Reduced engine power or 'limp home' mode activation
- Unresponsive accelerator pedal
Common Causes
- Defective or worn Throttle Position Sensor (TPS)
- Corroded or loose electrical connections at the TPS
- Fretting corrosion within the TPS connector
- Damage to the wiring harness leading to the TPS
- Accumulated carbon buildup on the throttle body affecting plate movement
- Internal fault within the Engine Control Module (ECM)
Frequently reported on: 2000-2006 Chevrolet Tahoe, 2000-2006 GMC Sierra 1500, 2000-2005 Cadillac DeVille, 2003-2007 Saturn Ion
Diagnostic Steps
- Utilize a scan tool to monitor the Throttle Position (TP) sensor voltage PID at various throttle angles, observing for signal dropouts or erratic readings.
- Perform a visual inspection of the throttle body assembly, ensuring the throttle plate moves freely without binding and checking for excessive carbon buildup.
- Inspect the electrical connector to the TPS for signs of corrosion, bent pins, or a loose fit.
- Verify the integrity of the wiring harness from the TPS to the ECM for chafing, cuts, or heat damage.
- With the ignition off, disconnect the TPS and use a multimeter to check for proper 5-volt reference and ground signals at the connector.
- If the sensor is suspected faulty after diagnostic testing, carefully remove and replace the Throttle Position Sensor, ensuring correct installation.
- After replacement, clear the diagnostic trouble code and perform an idle relearn procedure, which may involve specific key cycles or scan tool functions depending on the vehicle platform.
👨🔧 When to Call a Professional
If the throttle response remains erratic or the vehicle enters a 'limp home' mode after sensor replacement and an attempted idle relearn, it's time to consult a professional for advanced diagnostics. This could indicate a deeper ECM or wiring harness issue.
Commonly Asked Questions
How does the ECM determine the TPS is 'out of range'?
The ECM has defined minimum and maximum voltage parameters for the TPS signal. If the actual voltage falls below the minimum threshold (e.g., 0.2V at closed throttle) or exceeds the maximum (e.g., 4.8V at wide open throttle) under conditions where it should be within range, it flags P1120, indicating an anomaly in the sensor's operational window.
Can a dirty throttle body cause P1120?
While a dirty throttle body primarily affects idle quality by impeding throttle plate closure, it's less common for it to directly cause a P1120 'out of range' condition on the sensor signal itself. However, excessive carbon could potentially hinder the throttle plate's physical movement, leading to an incorrect mechanical position that the TPS reports, which the ECM might then interpret as an anomaly if it's outside expected learned values.
Is it safe to drive with a P1120 code?
Driving with P1120 is generally not recommended as it impacts the engine's ability to accurately control fuel delivery and ignition timing based on throttle input. This can lead to unsafe conditions such as sudden loss of power, stalling, or unintended acceleration, particularly if the sensor provides intermittent or erroneous data.
What is a 'limp home' mode and how does it relate to P1120?
'Limp home' mode is a protective strategy the ECM activates when it detects a critical sensor failure, like the TPS being out of range. The ECM will limit engine RPM and vehicle speed to a safe, reduced level to prevent further damage and allow the driver to reach a service center, relying on default values rather than potentially incorrect sensor data.
Do I need to reprogram the ECM after replacing the TPS?
For many GM systems, after replacing the TPS, it's essential to clear the Diagnostic Trouble Codes (DTCs) and often perform an idle learn or throttle body relearn procedure. This allows the ECM to recalibrate to the new sensor's signal and learn the new throttle plate's closed and wide-open positions for optimal drivability and to prevent recurring issues.