P039A Cylinder 1 Pressure Too Low

Some diesels have a combustion pressure sensor in the glow plug; the engine module uses it to control injection for each cylinder. This code concerns cylinder 1. P039A means the pressure measured in the cylinder is lower than expected while the sensor itself is electrically fine. No manufacturer document available to us describes the code; GM patents show how such an evaluation works: peak and minimum pressure are compared with limits and with a motoring pressure calculated from manifold pressure and compression ratio, and the ratio of measured to motoring pressure is compared between cylinders. A low value can mean weak combustion or compression – leaking valve, worn rings, leaking glow plug seat, an injector delivering too little – or a sensor drifting downwards. The usual consequence is rough running and more smoke.

Likely causes

Symptoms

• Check engine light on
• Lack of power, rough running, possibly misfire on this cylinder
• More smoke

Repair estimate

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

Related codes

References: patents.google.com, patents.google.com, law.cornell.edu.