Code
P1185
BUICK
P — Powertrain
Engine Oil Temperature Circuit
AI status
Completed
Completed
100%
Causes
- Damaged or failed engine oil temperature sensor (NTC thermistor)
- Open, shorted or corroded wiring/connectors in the EOT circuit
- Poor or missing ground or reference voltage to the sensor
- Contaminated or incorrectly seated sensor
- PCM internal fault or software issue
- Very low oil level or severely degraded oil affecting sensor reading
Symptoms
- Malfunction Indicator Lamp (MIL) / Check Engine light illuminated
- Incorrect oil temperature reading on dash (erratic or out-of-range)
- Cooling fan operation or engine control behavior affected by wrong temp data
- Hard starting, poor idle, reduced fuel economy or drivability issues in some cases
- Possible stored freeze frame and related data in PCM
What to check
- Retrieve codes and freeze-frame/live data with a scan tool; capture the EOT PID value
- Visually inspect EOT sensor connector, wiring harness and nearby components for damage, corrosion, or oil contamination
- Verify engine oil level and condition (do not rely on a bad sensor reading alone)
- Backprobe sensor connector to check reference voltage and signal voltage with ignition on and engine at operating temp
- Measure sensor resistance out of circuit (cold and hot) and compare to expected behavior (resistance decreases as temperature rises)
- Perform wiggle test on harness while monitoring live data to detect intermittent faults
Signal parameters
- EOT sensors are typically NTC thermistors: resistance decreases as temperature increases
- Expected sensor voltage typically varies between ~0.5 V (hot) and ~4.5 V (cold) depending on circuit design
- Expected resistance values depend on sensor design; verify with OEM specifications. Typical behavior: high resistance at ambient/cold, low resistance at hot engine temperature
- Reference circuit often uses a stable reference voltage (e.g., 5V) or a pull-up; verify correct reference at connector
Diagnostic algorithm
- Read and record stored trouble codes, freeze frame and live EOT PID data with a scan tool.
- Visually inspect the sensor, connector and wiring for damage, oil intrusion, corrosion, or loose terminals. Repair or clean as needed.
- Verify engine oil level and condition; top up if low and retest since low oil can change thermal contact to the sensor.
- With ignition ON (engine OFF) backprobe the sensor connector: confirm reference voltage and ground are present. If missing, trace wiring toward PCM, check fuses and grounds.
- Start the engine and monitor the EOT voltage or temperature PID while warming up. Confirm the reading moves smoothly from cold to operating temperature.
- Remove the sensor and measure its resistance at known temperatures (room temp and warmed). Compare trend to expected thermistor behavior. If resistance is open or out of expected range, replace the sensor.
- If sensor checks good but signal is incorrect at PCM connector, inspect wiring for short to ground or battery, repair wiring harness, or replace connector pins as needed.
- After repairs, clear codes and road test. If code returns and wiring/sensor pass tests, consider PCM diagnosis or reflash by a dealer.
- Safety: allow engine/oil to cool before removing sensor, and take care to avoid burns and oil spills.
Likely causes
- Connector corrosion or bent/worn pins at the sensor
- Broken wire insulation contacting engine ground or battery voltage
- Sensor failed due to age, thermal cycling, or oil contamination
- Loose or missing ground near the sensor or PCM
- Aftermarket parts or incorrect replacement sensor
Fault status
Status
Engine Oil Temperature Circuit fault detected (open/short/implausible signal). Inspect sensor, wiring, grounds and PCM.
Repair difficulty: Medium
Diagnostic time: 0.5 - 2.0 hours
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