P2039
Reductant Injection Air Pressure Sensor A Circuit Low
Diesel exhaust aftertreatment (SCR)
Generic OBD-II
Medium
What it means
P2039 means the aftertreatment controller read an abnormally low voltage from the reductant (DEF) injection air pressure sensor 'A'. On air-assisted diesel SCR systems this sensor monitors the air used to atomize the reductant. A low-circuit fault usually comes from wiring or a failed sensor, and can lead to a power derate as the system protects emissions compliance.
Medium
Can I keep driving? Usually driveable at first, but many diesels reduce power (derate) if SCR dosing is disabled — repair it before the vehicle limits performance.
Common symptoms
What you'll notice
- Check-engine light on
- SCR / DEF or emissions-system warning message
- Possible reduced power (engine derate) if dosing is disabled
- Increased NOx / failed emissions readiness
Common causes
Most likely first
- Short-to-ground or open in the reductant air pressure sensor wiringLikely
- Failed reductant injection air pressure sensor 'A'Common
- Corroded or damaged sensor connectorCommon
- Air-assist line or pump fault affecting sensed pressureLess often
- Faulty PCM/aftertreatment control module — less commonLess often
How to diagnose & fix
Work through it in order
- Read freeze-frame data and inspect the air pressure sensor connector and wiring for shorts or corrosion.
- Check the sensor's reference voltage and ground; look for a short to ground pulling the signal low.
- Inspect the reductant air-assist line and pump for leaks or blockage.
- Test and replace the air pressure sensor if out of spec. Clear and re-test.
FAQ
Quick answers
Is it safe to drive with P2039?+
Short-term usually, but diesel emissions systems often derate power when SCR dosing can't run correctly. Repair it before the vehicle limits performance or restarts.
Can I just clear P2039?+
It returns until the circuit fault is fixed, and clearing does not stop an emissions-related derate from recurring.
Related codes
Often seen with P2039
Read & clear P2039 yourself
Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.
✓ Verified name — SAE J2012 / ISO 15031-6Guidance reviewed by OBD DashX