What does it mean?
Range/performance is a plausibility fault rather than a wiring failure: the combustion pressure sensor on cylinder 6 is still sending a signal inside its normal voltage window, but the shape or level of that signal does not agree with what the engine is actually doing. The control module cross-checks each pressure trace against crankshaft position, engine speed, load, injection or ignition timing and the traces coming from the other cylinders.
When the peak arrives at the wrong crank angle, the baseline drifts, the amplitude no longer scales with fuelling, or the reading stays fixed while conditions change, the value is rejected as unreasonable. Soot packing the pressure channel, a sensor tightened to the wrong torque or a slowly drifting sensing element all produce a signal that looks valid on a meter yet is wrong in context. Once the trace is rejected, cylinder 6 is dropped from combustion feedback and the code is stored.
Symptoms
- MIL on with a freeze frame usually recorded under mid or high load
- Uneven running and a slight vibration at idle that comes and goes
- Combustion noise or a knocking sound traceable to one part of the engine
- Reduced power, flat spots and a lazy response when accelerating
- Increased fuel consumption and, on diesels, darker exhaust under acceleration
- Occasionally harsh or delayed particulate filter regeneration on diesel vehicles
Likely causes
- Carbon and soot deposits blocking the pressure passage in the sensor tip, damping and delaying the trace
- Sensor installed at incorrect torque or without its sealing washer, so mounting preload distorts what it reports
- Drift and reduced sensitivity in an aged piezo element
- Combustion leak past the sensor seat, or a leaking injector copper washer nearby, bleeding pressure away from the sensor
- A genuine mechanical deviation on cylinder 6 - low compression, a worn injector, valve clearance out of specification - which the module reads as an implausible trace
- High resistance in the signal or reference circuit adding an offset to the measured voltage
- Crankshaft or camshaft position signal slightly out, so pressure peaks are referenced to the wrong crank angle
How to fix it
- Record the freeze frame and note load, coolant temperature and engine speed at the moment the fault set
- Graph the live pressure traces of all cylinders side by side; a delayed, flattened or offset trace on cylinder 6 against healthy neighbours confirms the complaint
- Measure the signal and reference circuits for added resistance and check the connector for spread or corroded pins that lift the offset
- Remove the sensor or pressure sensing glow plug and inspect the tip for packed soot, a cracked sealing face or a missing washer
- Verify installation torque and sealing washer condition on refitting, since preload directly changes what the element reports
- Run a relative compression test and a cylinder contribution test on cylinder 6 to separate a false reading from a real mechanical shortfall
- Check crankshaft and camshaft correlation before replacing anything, then clear the code and road test under the conditions from the freeze frame