What does it mean?
Besides absolute pressure values, the module studies how much the cylinder 3 pressure curve changes from one combustion cycle to the next, because a running engine always shows a small and characteristic amount of cycle to cycle variation. This code is stored when that variation falls below the calibrated minimum, so the trace looks unnaturally flat and no longer moves with the combustion event.
A flat trace rarely means an exceptionally good cylinder; it usually means the signal path is damped and the sensor is not seeing the real pressure pulse. The common mechanisms are a blocked pressure channel, a sensor that is not clamped tightly against the chamber, or an element that has lost sensitivity. Because a damped trace cannot support closed loop combustion control, the module reverts to open loop values for cylinder 3 and lights the MIL (check engine lamp).
Symptoms
- MIL (check engine lamp) on, frequently with little obvious change in the way the engine drives
- Combustion note slightly harsher because real time correction for that cylinder is no longer applied
- Idle a little less refined than the driver is used to
- Small loss of throttle response and a modest rise in fuel consumption
- More smoke than usual on a cold start on diesel applications
- Cylinder balance or combustion performance codes appearing on the same cylinder over time
Likely causes
- Carbon packed into the pressure channel in front of the sensing element, filtering out rapid pressure changes
- Sensor tightened to the wrong torque or fitted with a failed sealing washer, so the element does not see the full pulse
- Aged sensing element with reduced sensitivity producing a compressed, low amplitude output
- Wrong sensor variant installed, with an output range the calibration does not expect
- High resistance in the signal or low reference circuit acting as an unintended filter
- Corrosion or moisture in the connector damping the signal amplitude
- Module input drift, considered only after the sensor and its wiring have been checked
How to fix it
- Compare the live pressure traces of all cylinders at idle and under steady load and confirm that only cylinder 3 looks flat
- Check the compression and injector correction values for cylinder 3, because normal values rule out a genuine engine change
- Inspect the connector for corrosion and moisture and measure the resistance of the signal and low reference circuits back to the module
- Remove the sensor and inspect the tip and the pressure channel for carbon packing, and check the sealing washer and the bore for damage
- Verify the fitted sensor part number against the specification, because a substituted variant gives the wrong amplitude
- Refit or renew the sensor with a new seal at the specified torque, since torque affects how the element reads the pressure pulse
- Clear the codes and confirm on live data that the cycle to cycle variation of cylinder 3 now sits in the same band as the other cylinders