What does it mean?
An intermittent or erratic fault means the pressure signal from cylinder 4 is present most of the time but keeps dropping out, jumping, or changing in steps that combustion alone cannot explain. The engine control module samples the pressure trace many times per crank revolution, so a momentary break in the circuit or a noisy contact produces spikes and gaps the diagnostic strategy catches easily, even when a static voltage check looks perfect.
Vibration, thermal expansion around the cylinder head and moisture in a connector are the classic triggers, which is why the code tends to appear after a cold start, over rough roads, or once the engine reaches full temperature. Because the fault comes and goes, the module usually needs the pattern to repeat across several drive cycles before it commits the code and lights the MIL. Diagnosis has to be carried out with the fault reproduced, not at rest.
Symptoms
- MIL that comes on and later goes out by itself over successive trips
- Brief stumbles or a momentary shake that will not repeat on demand
- Occasional rough running that clears again within seconds
- Fault appearing mainly on rough surfaces, over bumps, or at one particular engine temperature
- Slight loss of power or a hesitation on acceleration the driver cannot reliably reproduce
- Fuel consumption creeping up without any single obvious event
Likely causes
- Loose, spread or backed-out terminal in the sensor connector at cylinder 4
- Harness insulation worn through at a rub point, making momentary contact with a bracket or the cylinder head
- Moisture, oil or coolant tracking inside the connector and breaking down intermittently as it heats
- Wire strands broken inside intact insulation, usually where the loom flexes near an engine mount
- Aging sensor element that drops out once the cylinder head reaches operating temperature
- Weak or corroded engine ground point letting electrical noise onto the low reference
- Signal wire routed too close to an ignition or injector circuit and picking up interference
How to fix it
- Read the freeze frame and note the temperature, speed and load at which the code sets, then work to reproduce those conditions
- Graph the live pressure signal for cylinder 4 while wiggling the connector and the harness section by section; a dropout on movement locates the fault immediately
- Warm the engine fully and repeat the wiggle test, since some faults only appear once the head has expanded
- Inspect the connector for terminal tension, corrosion, moisture and seal condition, and check pin retention by gently pulling each wire
- Follow the harness across the cylinder head looking for rub marks, hard spots and heat damage
- Clean and tighten the engine and module ground points before assuming a sensor fault
- Replace the sensor only where the signal still drops out with the harness proven good; refit at the specified torque, clear the code and re-test over the same route