What does it mean?
The module tracks how much the combustion pressure of cylinder 1 differs from one cycle to the next and uses that figure as a direct measure of combustion stability. This code is stored when the variation rises above the calibrated maximum, meaning that consecutive cycles in the same cylinder are producing noticeably different pressure curves.
That instability usually comes from something that changes between cycles, such as an injector that meters unevenly, air or water reaching the nozzle, or a cylinder whose compression is marginal. A noisy or unstable electrical signal can produce the same result without any real combustion problem, so the sensor and its wiring have to be cleared before mechanical work begins.
Whatever the source, the module can no longer rely on that cylinder for closed loop control, and it sets the MIL (check engine lamp).
Symptoms
- MIL (check engine lamp) on, frequently with misfire or contribution codes for the same cylinder
- Uneven idle with a beat that comes and goes rather than a steady rhythm
- Slight surging or an unsettled feeling when holding a constant speed
- Rougher running when cold that improves once the engine reaches temperature
- Small hesitations on acceleration and a modest rise in fuel consumption
- Occasional puff of smoke on diesel applications during load changes
Likely causes
- Injector on cylinder 1 delivering an unstable quantity because of a worn nozzle, a sticking needle or internal leakage
- Air or water in the fuel reaching that injector, causing individual cycles to burn differently
- Marginal or uneven compression in cylinder 1 from a leaking valve, worn rings or an early head gasket failure
- Faulty glow plug or weak ignition energy in that cylinder, producing partial combustion cycles while the engine is cold
- Fuel supply instability such as a failing pressure regulator, a restricted filter or a weak lift pump
- Excessive or uneven exhaust gas recirculation flow reaching that cylinder through a fouled intake port
- Noisy sensor signal from a loose terminal, a chafed wire or interference picked up by a poorly routed harness
How to fix it
- Read the freeze frame and note whether the instability appears cold, hot, at idle or under load
- Check the sensor connector and harness for loose terminals, corrosion and chafing, and watch the signal for noise while wiggle testing
- Compare the cycle to cycle variation of all cylinders on live data to confirm the problem is genuinely limited to cylinder 1
- Examine the injector correction and balance values for that cylinder and check fuel rail pressure stability while the fault is present
- Bleed and inspect the fuel system for air ingress, a restricted filter or a weak supply pump, and check the fuel for water contamination
- Test the glow plug or the ignition components for that cylinder if the instability is worst during a cold start
- Only when fuel and ignition are proven, perform a compression or leak down test on cylinder 1, then repair the cause found and verify the variation returns to the normal band