What does it mean?
Beyond peak values, the module also studies how much the pressure trace of cylinder 1 changes from one combustion cycle to the next, because a healthy engine always shows a small, characteristic amount of cycle to cycle variation. This code is stored when that variation falls below the calibrated minimum, which means the trace looks unnaturally flat and smooth.
A signal that no longer breathes with the combustion event is normally a measurement problem rather than a sign of an exceptionally good cylinder. The usual mechanism is a damped signal path: a blocked pressure channel, a sensor that is no longer seated tightly against the chamber, or an element that has lost sensitivity. Because the module cannot use a damped trace for closed loop combustion control, it drops back to open loop values for that cylinder and turns on the MIL (check engine lamp).
Symptoms
- MIL (check engine lamp) on, often with no obvious change in the way the engine runs
- Slightly harsher combustion note because timing for that cylinder is no longer corrected in real time
- Idle quality a little less smooth than the driver remembers
- Small loss of response and a modest rise in fuel consumption
- On diesel applications, more smoke than usual during a cold start
- Additional combustion balance or performance codes appearing on the same cylinder over time
Likely causes
- Soot or carbon packed into the pressure channel in front of the sensing element, filtering out the fast pressure changes
- Sensor not tightened to specification or a failed sealing washer, so the element does not see the full pressure pulse
- Aged sensing element that has lost sensitivity and now outputs a compressed, low amplitude signal
- Incorrect sensor variant fitted, with a different output range from the one the calibration expects
- High resistance in the signal or low reference circuit acting as an unintended filter on the signal
- Water or corrosion in the connector damping the signal and reducing its amplitude
- Module input drift, considered only after the sensor and the wiring have been checked
How to fix it
- Compare the live pressure traces of all cylinders at idle and at a steady load, and check whether only cylinder 1 looks flat
- Confirm compression and injector correction values for cylinder 1 are normal, which rules out a genuine engine change and confirms a sensor issue
- 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 part number of the fitted sensor against the specification for the engine, since a substituted variant produces the wrong amplitude
- Refit or renew the sensor to the correct torque with a new seal, since torque directly affects how the element reads pressure
- Clear the codes and confirm on live data that the cycle to cycle variation of cylinder 1 now sits in the same band as the other cylinders