Code
P1120
OLDSMOBILE
P — Powertrain
Throttle Position TP Sensor 1 Circuit
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UK: 21
EN: 71
RU: 33
AI status
Completed
Completed
100%
Causes
- Damaged or chafed wiring in the throttle body harness
- Poor or corroded connector at the TPS or ECM
- Failed/throttled TP sensor (potentiometer or hall element)
- Short to voltage or ground in the signal wire
- Poor 5V reference or sensor ground from the ECM
- Contaminated or mechanically worn throttle body or sensor linkage
Symptoms
- Check Engine Light (MIL) illuminated
- Rough or unstable idle
- Hesitation, stumble, or poor throttle response
- Surging or unpredictable engine speed during throttle changes
- Engine may enter limp-mode or have reduced power
- Difficulty starting or stalling at idle (possible)
What to check
- Retrieve freeze frame and live data with a scan tool; note TPS voltage at idle and during throttle travel
- Inspect connectors and wiring at the throttle body for corrosion, bent pins, or physical damage
- Backprobe TPS connector and measure reference (5V), ground, and signal with key ON engine OFF and while moving throttle
- Verify signal voltage changes smoothly from closed throttle to wide open throttle (no jumps or dropouts)
- Perform wiggle test on harness while observing live data for intermittent faults
- Check continuity and resistance of signal/ground/reference wires between TPS connector and ECM
Signal parameters
- Reference voltage: approximately 5.0 V (typically 4.75–5.25 V) from ECM
- Sensor ground: near 0 V (should be
- TP Sensor signal: typically ~0.5–1.0 V at closed/throttle idle and ~4.0–4.5 V at wide open throttle (linear increase with throttle opening)
- Typical smooth ramp with no sudden jumps or intermittent drops
- Sensor impedance varies by design; expect several kiloohms for potentiometer‑type TPS
Diagnostic algorithm
- Connect a scan tool, read freeze frame and pending codes; note engine conditions when fault occurred.
- Visually inspect throttle body, TPS connector and wiring for damage, contamination, loose pins or corrosion. Repair obvious issues.
- With key ON ENGINE OFF, backprobe connector: verify 5V reference present and stable, ground continuity, and signal voltage. If reference or ground missing, trace to ECM and repair wiring.
- Slowly open and close throttle while watching TPS signal on scan tool or multimeter: signal should change smoothly from ~0.5–1 V (closed) to ~4.0–4.5 V (WOT). Look for dead spots, jumps or dropouts.
- If signal is stuck or out of range, check for short to 5V or short to ground on the signal wire using a multimeter and continuity checks to other circuits.
- Perform continuity/resistance check of each wire between TPS connector and ECM connector to rule out open or high resistance. Wiggle harness during test to find intermittent faults.
- If wiring and connectors are good but signal abnormal, remove and bench-test or replace the TPS per service manual. Re‑test after replacement.
- If new TPS and wiring good but fault returns, consider ECM input circuit fault — inspect board connectors and grounds; consult manufacturer guidance before replacing ECM.
- Clear codes and perform road test/functional test, monitoring live TPS data for proper behavior and to confirm the repair.
- Document repairs and verify no other related DTCs remain.
Likely causes
- Wiring damage or connector corrosion at the throttle body harness
- Failed TPS unit
- Shorted or open signal/ground/reference wire between TPS and ECM
- Contamination or mechanical binding at the throttle shaft/plate
Fault status
Status
P1120 — Throttle Position (TP) Sensor 1 Circuit. ECM detected an open, short, intermittent, or out‑of‑range signal from TPS #1. Inspect wiring, connectors, sensor output, and 5V reference/ground.
Repair difficulty: Medium
Diagnostic time: 0.5-2.0 hours
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