What does it mean?
The module uses the difference between consecutive combustion cycles in cylinder 2 as a direct measure of how stable the burn in that cylinder is. This code is stored when the variation exceeds the calibrated maximum, meaning successive cycles produce noticeably different pressure curves even though the operating conditions have not changed. Something that varies between cycles is the usual cause: an injector that meters unevenly, air or water reaching the nozzle, or a cylinder whose compression is marginal. An electrically noisy signal can produce the same result with no real combustion problem, so the sensor and its wiring must be cleared first. Either way the module can no longer use cylinder 2 for closed loop control and turns on 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 instead of a steady rhythm
- Slight surging or an unsettled feeling when holding a constant road speed
- Rougher running when cold that improves as the engine warms up
- Small hesitations on acceleration and a modest rise in fuel consumption
- Occasional puff of smoke on diesel applications when the load changes
Likely causes
- Injector on cylinder 2 delivering an unstable quantity because of a worn nozzle, a sticking needle or internal leakage
- Air or water in the fuel reaching that injector, so individual cycles burn differently
- Marginal or uneven compression in cylinder 2 from a leaking valve, worn rings or an early head gasket problem
- Faulty glow plug or weak ignition energy in that cylinder, producing partial combustion cycles while the engine is cold
- Unstable fuel supply from a failing pressure regulator, a restricted filter or a weak lift pump
- Uneven exhaust gas recirculation reaching that cylinder through a fouled intake port
- Noisy signal caused by a loose terminal, a chafed wire or interference picked up by a badly routed harness
How to fix it
- Read the freeze frame and note whether the instability shows up cold, hot, at idle or under load
- Check the connector and harness for loose terminals, corrosion and chafing, and watch the signal for noise during a wiggle test
- Compare the cycle to cycle variation of every cylinder on live data to confirm the problem really is limited to cylinder 2
- Examine the injector correction and balance values for that cylinder and check 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
- Test the glow plug or the ignition components for cylinder 2 if the instability is worst during a cold start
- Once fuel and ignition are proven, carry out a compression or leak down test on cylinder 2, repair the cause and confirm the variation returns to the normal band