Limit driving — lean mixture risks misfire damage. P0108 means the MAP sensor signal voltage is above the maximum acceptable level — the PCM sees a pressure that is impossibly high for current engine conditions. The fault is most commonly caused by the signal wire shorted to voltage, an open sensor ground, or a failed MAP sensor producing a fixed high output.
What P0108 means
The MAP sensor outputs a signal voltage proportional to intake manifold pressure across a 0.2 V to 4.9 V range under normal operation. P0108 is stored when the signal voltage exceeds the maximum acceptable threshold while conditions are inconsistent with a high-pressure reading and engine speed is between 600 and 3500 RPM with battery voltage above 10 V. It is a one-trip fault. Three clean trips are required to extinguish the MIL after the fault is repaired.
Symptoms
- Illuminated check engine light, sometimes with ETC warning
- Engine running lean — PCM calculates a low-load condition and reduces fuel delivery
- Lean misfire, rough idle or hesitation on acceleration
- Reduced power output from insufficient fuel delivery under load
- Poor cold start as the PCM underestimates required cold enrichment
Common causes
- MAP signal circuit (K1) shorted to a voltage source above the reference level
- MAP signal circuit shorted to the 5-volt supply wire (K856), causing a continuous near-5 V reading
- Open or excessive resistance in the sensor ground circuit (K900), leaving the signal floating high
- Failed MAP sensor internally shorted to its own reference, producing a fixed high output
- Cracked or kinked vacuum hose to the sensor port causing an actual pressure anomaly in certain conditions
Severity & driving advice
Severity: medium — Engine runs lean under load, risking misfire and catalyst overtemperature. Diagnose promptly.
Can I drive? Limit driving — lean mixture risks misfire damage.
Diagnostic approach
- Check 5-volt supply, signal and sensor ground with ignition on — With the MAP sensor connected and ignition on, verify: 5-volt supply pin reads 4.5-5.2 V to chassis ground; sensor ground pin reads below 0.1 V to battery negative; signal pin should vary with manifold pressure. A floating or high-resistance ground forces the signal output to rise artificially and is a leading cause of P0108.
- Check for signal circuit shorted to voltage — Disconnect the MAP sensor. With ignition on, measure voltage on the MAP signal wire (K1) at the harness connector. With the sensor unplugged, the PCM's internal pull-up holds this near the reference (approximately 4.5-5 V). A reading at battery voltage (12-14 V) confirms the circuit is shorted to a power source independent of the reference.
- Test sensor ground circuit continuity (K900) — With ignition off and both the sensor and PCM disconnected, measure resistance on the K900 sensor ground circuit from the MAP harness connector to the PCM connector. The reading must be below 5.0 ohms. Higher resistance from corroded terminals or broken wire strands prevents the signal from referencing correctly and pushes the output high.
- Check for signal-to-supply short (K1 to K856) — With both the sensor and PCM disconnected, measure resistance between the MAP signal circuit (K1) and the 5-volt supply circuit (K856) at the harness connector. A reading below 100 ohms confirms the wires are shorted together, causing the signal to track the reference voltage continuously near 5 V.
- Replace MAP sensor if all circuits test good — If the 5-volt supply is 4.5-5.2 V, the sensor ground is below 5 ohms, and there is no cross-short or short to voltage, the MAP sensor has an internal fault — replace it. After installation, confirm the signal voltage at idle is approximately 1-2 V (high vacuum) and rises toward 4.5 V as the throttle is opened, and reads near 4.5-4.9 V with ignition on, engine off (atmospheric pressure).
Make & model notes
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