Code
P1121
GM
P — Powertrain
TP Sensor Circuit Intermittent High Voltage
Views:
UK: 38
EN: 91
RU: 47
AI status
Completed
Completed
100%
Causes
- Faulty throttle position sensor (TPS)
- Short in signal wire to battery voltage (12V)
- Intermittent open/poor connection at TPS connector (corrosion, bent pin)
- Damaged/chafed wiring or pinched harness
- Poor sensor ground or reference 5V supply intermittent
- Contamination or mechanical damage in throttle body/sensor
Symptoms
- Check Engine Light (MIL) illuminated, P1121 stored
- Intermittent rough idle, surging or stalling
- Hesitation or poor throttle response
- Cruise control may not engage or may drop out
- Possible limp-home mode / reduced engine power
- Fault may be intermittent—symptoms come and go
What to check
- Read freeze frame and all stored codes; check for related TPS or throttle-body codes
- Monitor TPS signal voltage (and percent) with a scan tool during key-on and while operating throttle
- Backprobe TPS connector to verify reference 5V, signal, and ground while wiggling wiring
- Inspect connector for corrosion, bent pins, moisture, or debris
- Perform wiggle/strain test on harness from sensor to PCM looking for intermittent change
- Check wiring continuity and insulation; look for chafing against body or engine
Signal parameters
- Reference (Vref): ~5.0 V (supplied by PCM) — should be steady
- Signal voltage (closed throttle): typically ~0.2–1.0 V depending on vehicle/calibration
- Signal voltage (wide-open throttle): typically ~4.0–4.8 V
- Expected: smooth, monotonic change with throttle movement; no spikes above Vref
- Intermittent high condition: brief signal excursions above the expected max (≥~4.8–5.0 V) or transient spikes
Diagnostic algorithm
- Confirm code and capture freeze-frame data. Note operating conditions when code set.
- With a capable scan tool, monitor TPS voltage and throttle % during key-on engine-off and while revving/throttling. Try to reproduce the intermittent high reading.
- Visually inspect TPS connector and wiring harness for damage, corrosion, moisture, or repair splices. Repair any obvious damage.
- Backprobe the TPS connector: verify steady 5V reference, sensor signal, and good ground. Watch for voltage drops or spikes while wiggling the harness and operating the throttle.
- Use an oscilloscope if available to capture transient spikes on the signal line that a DMM may miss.
- If wiring or connector intermittency is found, repair (replace terminals, solder/heat-shrink, replace harness section) and retest until signal is stable.
- If wiring/connector and voltages are within spec but signal still intermittently high, replace the TPS/throttle-body assembly per service procedure and retest.
- If replaced sensor and wiring checks out and fault persists, inspect PCM connector and chassis ground locations. Only consider PCM replacement or reprogramming after eliminating wiring and sensor causes.
- Clear codes and verify repair by road test and re-scan multiple drive cycles to ensure P1121 does not return.
Likely causes
- Corroded/loose TPS connector pins or terminal spring causing intermittent contact
- Wiring chafing or intermittent short to 12V on the TPS signal conductor
- Failed TPS producing intermittent high output
- Intermittent loss of sensor ground or reference voltage
- Less likely: PCM driver intermittently sourcing voltage
Fault status
Status
P1121 — Throttle Position (TP) Sensor circuit reported intermittent high voltage. Inspect TPS, wiring, connector, reference 5V and ground; repair intermittent wiring or replace sensor as needed.
Repair difficulty: Medium
Diagnostic time: 1.0-2.5 hours
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