Cause
Faulty reductant injection air pressure sensor
On air-assisted reductant systems, the sensor watching the compressed air that atomises the fluid before it enters the exhaust. Only some systems use air assistance at all, which is worth establishing first — on an airless system this sensor does not exist and the code names something else entirely.
Atomisation is the reason the air circuit matters. Reductant delivered as a fine mist evaporates and reacts; delivered as a stream it lands on cold metal and crystallises into a hard deposit that blocks the injector and coats the catalyst face. So a fault in the air pressure reading is not a monitoring inconvenience — the module uses it to decide whether dosing is safe, and it will stop dosing rather than risk the deposit.
The circuit around it fails more often than the element. It sits in a hot, vibrating, salt-exposed position, so a chafed harness, a corroded pin or a cracked air line reaches it first. Check the air line for a leak or a blockage while the connector is off, because a genuinely low air pressure and a sensor misreporting it produce the same code and the line is the quicker of the two to rule out.
Typically a moderate job.
Codes this can cause (5)
Codes where this is a likely explanation first, rather than merely a possible one.
- P2037 Reductant Injection Air Pressure Sensor A Circuit Likely for this code Moderate
- P2038 Reductant Injection Air Pressure Sensor A Circuit Range/Performance Likely for this code Moderate
- P2039 Reductant Injection Air Pressure Sensor A Circuit Low Likely for this code Moderate
- P2040 Reductant Injection Air Pressure Sensor A Circuit High Likely for this code Moderate
- P2041 Reductant Injection Air Pressure Sensor A Circuit Intermittent Likely for this code Moderate
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