Code
P1185
HUMMER
P — Powertrain
Engine Oil Temperature Circuit
AI status
Completed
Completed
100%
Causes
- Open circuit (broken wire) in oil temperature sensor harness
- Short to voltage or short to ground in sensor wiring
- Corroded, loose or damaged sensor connector/terminal
- Failed/contaminated engine oil temperature sensor (thermistor)
- PCM/ECM internal fault or poor PCM/ECM ground/reference
Symptoms
- Check Engine Light (MIL) illuminated
- Incorrect or fluctuating oil temperature gauge/readout
- Longer warm-up enrichment or poor idle until PCM adapts
- Possible limp-home/derate or altered shift patterns if shared by transmission control
- Intermittent faults that may appear after vibration or heat soak
What to check
- Read freeze frame and freeze-data with a scan tool; record oil temperature PID and related sensors
- Visual inspection of oil temp sensor, connector, and harness for corrosion, oil contamination, chafing or heat damage
- Backprobe sensor connector and check for reference voltage, signal voltage, and ground with key ON (engine OFF) per service manual
- Measure sensor resistance at ambient and compare to expected thermistor characteristics (or check signal change while heating/cooling sensor)
- Check continuity between sensor connector and PCM pin(s); check for short to battery or ground
- Wiggle test harness while observing live data to reproduce intermittent fault
Signal parameters
- Typical sensor type: thermistor (NTC) — resistance generally changes with temperature (resistance decreases as temperature increases)
- Expected signal voltage range (approximate): 0.1–4.9 V depending on sensor design and oil temperature
- Open-circuit: very high resistance (often >50–100 kΩ) or signal voltage at one extreme (e.g., near 0 V or near reference voltage)
- Short-to-ground: signal near 0 V; short-to-voltage: signal near reference supply (often ~5 V)
- Normal operating oil temp PID values depend on engine; verify against ambient and warm engine conditions with scan tool
Diagnostic algorithm
- Connect a scan tool, read/record trouble codes and freeze frame, review live oil temperature PID and related PIDs (coolant temp, intake temp, etc.).
- Visually inspect the oil temp sensor and wiring harness for damage, oil intrusion, corrosion or poor connections. Repair obvious damage.
- With key ON (engine OFF), backprobe the sensor connector: verify reference voltage (if used), signal voltage, and ground continuity to PCM. Compare to service specs where available.
- Unplug the sensor and measure sensor resistance. Compare measured resistance to expected thermistor curve (or observe change while gently heating/cooling sensor).
- Check wiring continuity between the sensor connector and the PCM pin. Check for shorts to battery and ground with an ohmmeter. Repair any open/short.
- If wiring and connector are good but sensor readings are implausible, replace the oil temperature sensor. Retest and verify proper PID changes.
- If new sensor still shows fault and wiring is verified, inspect PCM grounds and power supplies. If PCM signals are missing or erratic, consider PCM diagnosis or replacement per factory procedures.
- Clear codes, perform a road or warm-up cycle, and confirm the code does not return and that oil temperature PID behaves normally.
Likely causes
- Corroded connector at sensor or harness rubbed through
- Damaged wiring due to heat, chafing, or engine movement
- Failed oil temperature sensor (common wear or oil contamination)
- Intermittent connector contact from oil or debris
- PCM/ECM failing to provide proper reference or ground
Fault status
Status
P1185 — Engine Oil Temperature Circuit malfunction: sensor circuit open/short/intermittent or implausible signal.
Repair difficulty: Medium
Diagnostic time: 0.5-2.5 hours
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Repair manuals
Manual library for HUMMER
138
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