P03D8: Cylinder 7 Pressure Variation Low

What it means

Cylinder-to-cylinder pressure variation on cylinder 7 is below the calibrated minimum — pressure stays unusually constant cycle-to-cycle, often a sign that the closed-loop combustion controller has lost authority or the sensor signal is filtered.

At a glance

• Usually lights the check-engine light
• Affects emissions; the car can fail an emissions test

How it works

The control module compares cylinder 7's pressure pattern across combustion cycles to assess how consistently that cylinder is firing. This code sets when the measured cycle-to-cycle pressure variation is unusually low, which can point to a sensor signal or processing problem rather than normal pressure changes.

Common causes

Most likely first.

1. Combustion pressure sensor signal fault
2. Pressure signal filtering or processing fault
3. Combustion-control system fault

Symptoms

• Check-engine light
• Rough or uneven engine running
• Reduced engine power

How to diagnose it

1. Record freeze-frame data and compare cylinder 7 pressure variation with other cylinders at the same operating conditions.
2. Check live data for an unusually flat cylinder 7 pressure trace while noting any rough running or power reduction.
3. Inspect the cylinder 7 pressure sensor connector and harness for damage, poor contact, or signs of moisture.
4. Check the sensor signal and its supply and ground circuits against the manufacturer's specification.
5. If the sensor and wiring check out, review available combustion-control data for filtering or processing faults using the manufacturer's diagnostic information.

Common fixes

• Repair poor connections or damaged wiring at the cylinder 7 pressure sensor.
• Replace the cylinder 7 pressure sensor if its output is abnormally flat and the circuit is sound.
• Correct a confirmed signal filtering or processing fault using the manufacturer's procedure.
• Repair a confirmed combustion-control fault affecting cylinder 7.

Can I keep driving?

Limit driving and avoid heavy loads until the cause is identified, especially if the engine runs unevenly or loses power. Continued operation with unreliable combustion feedback can worsen drivability and emissions.