Code
P1189
HUMMER
P — Powertrain
Engine Oil Pressure (EOP) Switch Circuit
AI status
Completed
Completed
100%
Causes
- Faulty Engine Oil Pressure (EOP) switch
- Damaged, corroded or disconnected wiring/connector to the EOP switch
- Short to power or short to ground in the switch circuit
- Low engine oil level or extremely low oil pressure
- Poor ground or blown fuse in related circuits
- Faulty Powertrain Control Module (less common)
Symptoms
- Malfunction Indicator Lamp (MIL) or oil warning lamp illuminated
- Oil pressure warning message on dash
- Oil pressure gauge reading erratic or stuck
- Possible engine protection actions (reduced power) on some vehicles
- No other drivability symptoms until oil pressure is actually low
What to check
- Check engine oil level and oil condition; top up if low
- Scan for related DTCs and capture freeze frame/data
- Visually inspect EOP switch connector and wiring for corrosion, damage, or loose pins
- Check relevant fuses and grounds for continuity
- Backprobe the switch circuit and monitor signal while cranking/idle
- Wiggle test harness while monitoring signal for intermittent faults
Signal parameters
- Switch type: typically a 2‑ or 3‑terminal pressure switch that changes state at a low oil pressure threshold (manufacturer-specific).
- Normal behavior: switch closes (logic = low or ground) when oil pressure is below threshold; opens when pressure is normal. Exact threshold varies by engine (commonly single‑digit to low‑tens psi).
- When closed: continuity near 0 Ω between switch terminal and ground (or PCM input reads
- When open: circuit shows high resistance/OL; PCM input typically pulled to reference voltage (~5 V) or logic high depending on design.
- Typical test voltages: expect near 0 V at the PCM pin when switch is closed; near reference voltage (often ~5 V) when open. Confirm with vehicle-specific wiring diagrams.
Diagnostic algorithm
- Retrieve freeze frame and all stored/related DTCs. Note engine conditions when code set.
- Verify oil level and condition. If oil is severely low or contaminated, correct and retest before electrical diagnosis.
- Perform a visual inspection of the EOP switch, connector, and wiring harness for damage, corrosion, or pin push‑out.
- With key on (engine off), backprobe the EOP switch connector and monitor voltage/continuity. Compare to expected open/closed values from manufacturer data.
- Crank the engine or run to idle (observe safe conditions) while monitoring the switch signal. The switch state should change only if oil pressure crosses the threshold.
- If the signal is incorrect or intermittent, disconnect the switch and measure continuity from the harness to the PCM connector. Check for shorts to power or ground and for open circuits.
- Perform a wiggle test on the harness while observing the signal to find intermittent breaks.
- If wiring and connector test good, bench-test or replace the EOP switch and retest. Reinstall and clear codes.
- If fault persists after switch replacement and wiring verified, test PCM input circuit for correct pull‑up/pull‑down and replace or reprogram PCM only after confirming module fault with advanced diagnostics.
- After repair, clear codes, perform functional test (start/run, road test if needed) and monitor data to confirm the issue is resolved.
Likely causes
- Corroded/loose connector at the EOP switch
- Open or shorted signal wire between switch and PCM
- Failed EOP switch (mechanical/electrical)
- Low oil level causing actual low pressure and switch closure
- Intermittent harness damage near flex points or engine mount
Fault status
Status
Engine Oil Pressure (EOP) Switch Circuit — Open/Short/Intermittent detected by PCM
Repair difficulty: Medium
Diagnostic time: 0.5-1.5 hours
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