What does it mean?
Variation describes how much the pressure signal changes - between the compression phase and the combustion peak, and from one cycle to the next - and this code is set when that change on cylinder 7 is smaller than it should be.
A healthy cylinder produces a lively trace that rises and falls with every stroke and shifts when fuelling or load changes; a trace that stays almost flat suggests the sensor is being damped or is no longer following pressure, or that the cylinder is producing very little combustion at all. Soot packed into the pressure channel is the classic cause, because it acts like a filter and smooths the peaks away.
The engine management module runs a plausibility check on trace amplitude precisely so a lazy or blinded sensor cannot quietly feed wrong data into combustion control. Once the variation falls below the calibrated window, cylinder 7 is excluded from feedback and the code is stored.
Symptoms
- MIL on, sometimes with no strong driving complaint at first
- Idle that feels slightly uneven or hollow rather than badly rough
- Reduced power and a duller response as the module works from base maps for that cylinder
- Combustion noise changing character, often quieter and flatter on the affected cylinder
- Fuel consumption creeping upward
- On diesels, less consistent smoothness during regeneration or under light load
Likely causes
- Soot or carbon packed into the pressure channel of the sensor tip on cylinder 7, damping the trace
- Aged piezo element with reduced sensitivity, still producing a signal but with far less amplitude
- Sensor not seated properly, or fitted with the wrong or a reused sealing washer, so preload is lost
- Very weak combustion in that cylinder from a failed injector, a dead coil or badly worn valvetrain
- High resistance in the signal circuit attenuating the returning signal
- Signal ground or shield problem allowing the trace to be filtered or clipped
- Charge amplifier or sensor conditioning circuit fault where the system uses one
How to fix it
- Read the freeze frame, then graph the pressure traces of all cylinders together and compare peak-to-peak amplitude
- Watch how the trace on cylinder 7 responds to a snap throttle; a trace that barely changes points at a blinded or damped sensor
- Measure signal circuit resistance and check connector terminals for corrosion that could attenuate the signal
- Remove the pressure sensor or pressure sensing glow plug and examine the tip and pressure passage for hard carbon, cleaning it only by the method the manufacturer permits
- Confirm the sealing washer is new and correct and that installation torque is exactly to specification, since preload sets the sensor's sensitivity
- Verify the cylinder is genuinely burning fuel properly with a contribution test, an injector check and a compression test before blaming the sensor
- Fit a new sensor where amplitude stays low with a proven circuit and a healthy cylinder, then clear the code and confirm the trace on a load test