What does it mean?
Circuit high is set when the signal from the cylinder 8 combustion pressure sensor sits above the highest voltage the engine control module will accept, typically close to the 5-volt reference level, and stays there through conditions in which it should be moving. A signal stuck at the top of its range points to an open low reference, a signal line touching battery voltage or the reference wire, or an internal fault leaving the sensor output biased high. The module cannot separate a genuine pressure peak from a pegged line, so it discards the input rather than acting on it. Cylinder 8 is dropped from combustion feedback control, the MIL comes on, and the module falls back on its base fuelling and timing maps.
Symptoms
- MIL on, with the code often returning shortly after being cleared
- Uneven idle and mild vibration felt through the body of the car
- Rougher combustion noise and, on diesels, a more pronounced knock at low speed
- Loss of power, delayed throttle response, or entry into limp mode on some vehicles
- Poorer fuel economy once the module stops trimming individual cylinders
- Reduced cold start quality where the sensor shares the glow plug housing
Likely causes
- Open or high-resistance low reference circuit at the sensor, letting the signal float up toward the reference voltage
- Signal wire shorted to the 5-volt reference wire inside a damaged harness section or a wet connector
- Signal wire shorted to battery voltage where the loom passes close to a power feed
- Open signal circuit between sensor and module, which many modules report as a high reading
- Internally failed sensor element stuck at full output
- Corroded or backed-out terminal in the sensor connector raising circuit resistance
- Engine control module input fault, considered only once the harness and sensor have tested good
How to fix it
- Read the freeze frame, then check whether other cylinders show the same pattern, which points at a shared ground rather than one sensor
- Back-probe the sensor connector at cylinder 8 with the engine running and confirm the signal really is pegged high
- Unplug the sensor and check the low reference for continuity to module ground; an open ground is the most common cause and is repaired, not replaced
- Verify the signal wire is not shorted to the 5-volt reference or to battery voltage with the module connector disconnected
- Check signal wire continuity end to end, since an open line reads high on most module inputs
- Inspect terminals for corrosion, spreading and push-out, replacing individual pins where the connector allows
- Fit a new pressure sensor or pressure sensing glow plug only after the circuit is proven, torque it correctly, and confirm a live pressure trace before releasing the vehicle