P0397 Cylinder 1 Pressure Sensor Circuit 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. P0397 means the sensor signal stays below its lower limit. GM's European 2.0 CDTI diesel (Saab 9-5, 2010) sets it when the signal is below the threshold for more than 2 s with the engine running, checked once per drive cycle, with the check engine light after two trips; the displayed maximum combustion pressure then reads 0 kPa instead of the normal 0–9,000 kPa. Saab names an open or grounded 5 V reference and a signal wire shorted to ground as causes; the glow plug should measure 3–12 Ω between reference and ground. Without the signal, combustion control for this cylinder is switched off, which is usually barely noticeable.

Likely causes

Symptoms

• Check engine light on (often after two trips)
• Usually barely noticeable – combustion control for the cylinder is switched off
• Possibly rougher running, more noise or slightly higher consumption

Repair estimate

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

Parts involved

Related codes

References: saabwisonline.com, law.cornell.edu, borgwarner.com.