Code
P1187
HUMMER
P — Powertrain
EOT Sensor Circuit Low Voltage
AI status
Completed
Completed
100%
Causes
- Failed EOT (engine oil temperature) sensor
- Short to ground in the EOT sensor harness
- Corroded, loose, or damaged connector at the sensor or PCM
- Open or high-resistance wiring between sensor and PCM
- Poor or missing ground or reference voltage to the sensor
- PCM internal fault (rare)
Symptoms
- Possible incorrect oil temperature reading on cluster or scan tool
- Service engine light (MIL) may be illuminated
- Cold/hot adaptive strategies may be affected (hard/rough cold start, altered shift or fueling behavior)
- Rarely, limp-home mode or reduced performance depending on OEM logic
- No start is unlikely from this code alone
What to check
- Retrieve freeze frame and live data for EOT from a scan tool; note stored conditions
- Visually inspect EOT sensor connector and wiring for corrosion, damage, or pin push-out
- Check battery voltage and chassis/engine grounds before testing
- Backprobe the sensor harness to observe signal voltage with key ON and engine running
- Measure continuity and resistance between sensor connector and PCM pin
- Disconnect sensor and check harness reference voltage at connector (should read pull-up/reference voltage from PCM)
Signal parameters
- Typical thermistor-style oil temp sensor voltage range (varies by vehicle): approximately 0.5–4.5 V across operating temperatures
- Low-voltage condition: signal < ~0.3 V (indicative of short to ground or failed sensor)
- Reference/pull-up voltage at harness (key ON): typically ~5 V (check OEM spec for exact value)
- Normal sensor behaves as resistance that changes with temperature; verify resistance vs. temperature per service manual
Diagnostic algorithm
- Connect a scan tool and record freeze frame and live EOT sensor values. Confirm P1187 is current or stored and note conditions when it set.
- Perform a visual inspection of the EOT sensor, connector, wiring, and nearby routing. Look for chafing, pin corrosion, or oil contamination.
- With key ON (engine OFF), backprobe the sensor connector: measure signal voltage at the harness. If signal is low (
- Disconnect the sensor and measure reference/pull-up voltage at the harness pin. If reference voltage is absent or low, check PCM power/ground and related fuses; if present, continue.
- Measure continuity/resistance from the harness signal pin to the PCM pin. Repair any opens or excessive resistance.
- Check sensor resistance with the sensor removed. Compare resistance at a known temperature to the specification in the service manual. Replace sensor if out of spec.
- If wiring and sensor are good, inspect PCM connector for damage and consider PCM bench test or replacement only after ruling out harness/sensor faults.
- After repairs, clear codes, perform function check and verify the EOT reading returns to expected range while monitoring live data. Test drive to confirm the code does not return.
- Safety note: avoid removing or testing the sensor on a hot engine; allow the engine to cool to prevent burns and oil spills.
Likely causes
- Corroded/loose connector at EOT sensor
- Damaged wiring harness with short to ground
- Faulty EOT sensor element
- Intermittent harness connection (broken wire under insulation)
Fault status
Status
PCM detected low voltage on the Engine Oil Temperature (EOT) sensor circuit and stored DTC P1187. Possible causes include short to ground in the sensor circuit, failed EOT sensor, or poor connector/circuit integrity. Further testing of sensor, harness, and PCM required.
Repair difficulty: Medium
Diagnostic time: 0.5-2.0 hours
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