What does it mean?
In the cylinder 4 pressure sensing circuit the module supplies both the reference voltage and the return path, and it expects the signal to remain inside a narrow window whatever the engine is doing. This code is stored when the signal stays above the calibrated maximum for longer than the monitor allows, a level that a correctly connected sensor cannot produce.
The most common mechanism is a lost return path, because without a solid low reference the signal line simply climbs toward reference voltage. The alternatives are contact with a higher voltage circuit or an internal break in the sensor that leaves the module input resting on its own pull up. With the reading out of range the module abandons cylinder 4 combustion feedback, applies default fueling and turns on the MIL (check engine lamp).
Symptoms
- MIL (check engine lamp) on with a fault recorded for the cylinder 4 pressure sensor
- Idle noticeably rougher and the engine sounding harder than normal
- Difficult cold starting and heavier smoke on the first start of the day
- Reduced power because combustion feedback from that cylinder is ignored
- Increased fuel consumption and, on some applications, limp mode
- Uneven pull that the driver notices most clearly under light acceleration
Likely causes
- Low reference circuit open or high in resistance, letting the signal float to the top of its range
- Signal wire shorted to the five volt reference or to battery voltage inside a damaged harness section
- Broken signal conductor so that only the internal pull up of the module is measured
- Internally open sensing element or a fractured joint inside the sensor body
- Corroded terminals adding series resistance that pushes the reading upward
- Moisture or road salt in the connector creating a leakage path from a powered pin to the signal pin
- Module input fault, considered last and only after the reference and ground circuits have been verified
How to fix it
- Read the freeze frame and check for pressure sensor codes on other cylinders that would move the diagnosis to a shared circuit
- Inspect the connector for corrosion, moisture, a damaged seal and pushed back terminals, and check the harness where it passes near hot components
- With the ignition on and the sensor connected, backprobe the signal line and confirm the value really is at the top of its range
- Unplug the sensor and measure the low reference to ground on the harness side, since high resistance here is the most frequent cause
- Verify continuity of the signal wire back to the module and confirm it is not shorted to the reference or to a powered circuit
- Fit a known good sensor of the same specification if one is available and see whether the fault follows the sensor
- Complete the repair, clear the memory and confirm on a full warm up and road test that the cylinder 4 signal stays inside its normal window