Code
P1188
SATURN
P — Powertrain
EOT Sensor Circuit High Voltage
AI status
Completed
Completed
100%
Causes
- Open or damaged wiring between EOT sensor and ECM
- Corroded, loose or contaminated sensor connector or terminals
- Failed EOT sensor (open or out-of-spec resistance)
- Poor ground or missing reference voltage at sensor circuit
- ECM input fault (less common)
- Intermittent connection due to vibration or damaged harness
Symptoms
- MIL (Check Engine Light) illuminated
- Incorrect oil temperature gauge or no oil temp reading (if displayed)
- Possible poor cold/hot enrichment behavior or drivability complaints if engine management uses EOT
- Fault present on key-on or after short drive
What to check
- Read freeze frame and code status with a scan tool; note any related codes
- Visually inspect the EOT sensor connector and wiring for damage, corrosion, pin push-out, or oil contamination
- Back-probe the sensor connector to confirm presence of reference voltage (typically 5V) and good ground at key-on
- Measure signal voltage at the harness with key-on and while changing oil temperature (expect voltage to change with temperature)
- Measure EOT sensor resistance at ambient temp and while warming/cooling the sensor to confirm thermistor behavior
- Perform wiggle test on wiring while monitoring live data for intermittent changes
Signal parameters
- Sensor type: thermistor (NTC) — resistance decreases as temperature increases
- Expected behavior: resistance changes with temperature; signal voltage should vary between low and near reference (approx. 0.1–4.9 V) as oil temp changes
- High-voltage fault: signal voltage near reference supply (approx. >4.5 V) indicating open or very high resistance
- Typical test: cold = higher resistance, warm/hot = lower resistance (compare to vehicle/service manual values)
Diagnostic algorithm
- Confirm code and note freeze frame data with a scan tool. Check for related coolant or temp sensor codes.
- Visually inspect the EOT sensor connector, pins and wiring harness for damage, oil contamination, corrosion, or disconnected terminals.
- With ignition ON (engine OFF), back-probe connector: verify ECM reference voltage (≈5V) and ground presence. If reference or ground missing, trace and repair wiring.
- Measure the sensor signal voltage at key-on and while warming/cooling the sensor. High/static voltage near reference suggests open circuit or failed sensor.
- Disconnect sensor and measure sensor resistance across its terminals at ambient. Then heat (warm water) and cool (ice/water) to confirm resistance changes. If open or not changing, replace sensor.
- If sensor tests good, perform continuity and resistance checks from sensor connector to ECM pin to locate open/short. Repair any damaged wiring or connectors.
- After repairs or sensor replacement, clear codes and perform a test drive or warm-up to verify the code does not return and the EOT reading behaves normally.
- If wiring and sensor are good but the fault persists, consult manufacturer diagnostic flow for possible ECM fault and perform ECM testing or replacement as directed.
Likely causes
- Open/damaged harness or connector at the EOT sensor
- Failed EOT sensor (open/high resistance)
- Corroded/loose connector terminals
- Poor ground or missing 5V reference
Fault status
Status
P1188 — EOT Sensor Circuit High Voltage: The engine oil temperature sensor circuit is reporting a voltage higher than expected (open/high resistance). Inspect sensor, connector, wiring, and reference/ground.
Repair difficulty: Medium
Diagnostic time: 0.5-2.0 hours
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