What does it mean?
This code is set when the pressure trace from cylinder 4 swings far more from one combustion cycle to the next than the calibration allows. Some cycle-to-cycle variation is normal, but a large spread means the charge is burning inconsistently - strong on one cycle, weak or partial on the next - or that the sensor is delivering a noisy signal that only looks unstable.
Partial misfire, an injector metering erratically, too much recirculated exhaust, poor fuel quality and marginal compression are the usual mechanical reasons; electrical noise on the signal or reference circuit is the usual false one. Because unstable combustion upsets torque delivery and emissions, the engine control module stops trusting cylinder 4 for feedback control, lights the MIL, and may bias fuelling toward a safer setting.
Symptoms
- MIL on, frequently with a misfire code for the same cylinder
- Idle that hunts or surges slightly instead of settling
- Occasional stumble or jerk when pulling away or cruising at light load
- Combustion noise varying from smooth to harsh over a few seconds
- Reduced power, higher fuel consumption and unstable exhaust smell or smoke
- Vibration that changes with load rather than staying constant
Likely causes
- Injector on cylinder 4 metering inconsistently through deposits, wear or an intermittent electrical fault
- Partial misfire from a weak coil, a worn spark plug or a poor plug seal on gasoline engines
- Excessive or uneven EGR flow from a sticking valve or a fouled cooler, destabilising the burn
- Borderline compression, sticking valves or valve clearance out of specification
- Poor quality, contaminated or water-affected fuel
- Electrical noise or an intermittent contact on the sensor signal or ground circuit making a stable burn look erratic
- Vacuum or boost leak local to that cylinder's intake runner, changing the charge cycle by cycle
How to fix it
- Start with the freeze frame, then look for accompanying misfire, injector or fuel trim codes that narrow the search quickly
- Watch the live pressure trace on cylinder 4 over many cycles and compare the spread with the other cylinders
- Check injector correction and return values, then swap the injector or coil with a neighbouring cylinder to see whether the instability moves with it
- Inspect the EGR valve and cooler for sticking or heavy fouling and compare actual flow with the commanded value
- Test compression and leak-down on cylinder 4 and confirm valve clearances where they are adjustable
- Rule out electrical noise by inspecting the sensor ground, shield and connector, and by wiggle-testing the harness while graphing the signal
- Correct the fuelling, ignition or mechanical cause found, then clear the code and confirm the cycle-to-cycle spread returns to normal on a road test