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 3. P03AA means the sensor signal is electrically fine but does not fit the expected value. No manufacturer document available to us describes the code; a GM patent shows how such a check works: in overrun the measured peak pressure is compared with a compression pressure calculated from manifold pressure and compression ratio, and the pressure values are also compared with limits and with crankshaft speed fluctuation. GM's European diesel in the Saab 9-5 uses the peak pressure in overrun anyway to find top dead centre. Typical causes are sensor drift from combustion deposits or a glow plug that leaks or is wrongly torqued. CARB requires a two-sided plausibility check of input sensors where feasible.
Medium severity. Can I keep driving?If the car is running normally, drive in moderation to a repair shop. If it loses power or another warning appears, do not continue.
Cylinder 3 is numbered according to the manufacturer: check your engine's manual before working on it.
Symptoms
- Check engine light on
- Combustion control for the cylinder off – slightly rough running, more noise
- Possibly more smoke or consumption
Most likely causes
- Sensor in the glow plug drifting (deposits, ageing)
- Glow plug leaking or wrongly torqued (pressure transmitted wrongly)
- Contact resistance in connector or ground
- Actual compression pressure differs (valve, rings)
What to check, in order
Start with the simplest. You can tick off each step.
Good to know
- Repair difficulty: medium
- It affects emissions.
What each character of P03AA means
- P: Powertrain: engine and transmission
- 0: Generic code: same on every make
- 3: Ignition and misfires
- AA: Specific fault in that system
Frequently asked questions
Can I keep driving with code P03AA?
If the car is running normally, drive in moderation to a repair shop. If it loses power or another warning appears, do not continue.
What causes code P03AA?
The most common causes are: sensor in the glow plug drifting (deposits, ageing); glow plug leaking or wrongly torqued (pressure transmitted wrongly); contact resistance in connector or ground.
Related codes
- P03A9Cylinder 3 Pressure Sensor Circuit
- P03ABCylinder 3 Pressure Sensor Circuit Low
- P03ACCylinder 3 Pressure Sensor Circuit High
- P03ADCylinder 3 Pressure Sensor Circuit Intermittent/Erratic
- P0396Cylinder 1 Pressure Sensor Circuit Range/Performance
- P03A0Cylinder 2 Pressure Sensor Circuit Range/Performance
Sources
- law.cornell.edu: 13 CCR 1968.2
- patents.google.com: US7726281
- saabwisonline.com: dtc p0399 p039c p039d p03a3 p03a6 p03a7 p03ad p03b
- borgwarner.com: pressure sensor glow plugs
Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.