P2039 - Reductant Injection Air Pressure Sensor 'A'' Circuit Low
System
Powertrain (Engine & Gearbox)
Technical Description
The control module has detected a fault in the Reductant Injection Air Pressure Sensor 'A'' Circuit Low (P2039). The signal is below the lower threshold, typically from a short to ground or an internally shorted sensor.
Multimeter Test Points & Nominal Values
| Test Point / Component | Measurement | Nominal Target Range | Test Condition |
|---|---|---|---|
| Reductant Injection Air Pressure Sensor 'A'' Circuit Low – Coil Resistance | Resistance (Ω) | 3 - 30 Ω (typical) | Connector disconnected at 20°C |
| Reductant Injection Air Pressure Sensor 'A'' Circuit Low – Duty Cycle | PWM (%) | 5 - 95 % (modulates with target) | Engine running, actuator in control |
| Reductant Injection Air Pressure Sensor 'A'' Circuit Low – Supply Voltage | Voltage (V) | 11.8 - 14.4 V | Ignition ON at connector |
Connector disconnected at 20°C
Engine running, actuator in control
Ignition ON at connector
Oscilloscope Waveform Analysis
2 ms/div, 20 V/divPossible Causes
Short to ground on the signal wire or an internal short circuit within Reductant Injection Air Pressure Sensor 'A'' Circuit Low
Channel A:Verify dedicated wiring pins and signal pair for circuit A against the schematic.
Chafed or pinched wiring harness along the signal path to Reductant Injection Air Pressure Sensor 'A'' Circuit Low
Internal mechanical wear or carbon deposits inside Reductant Injection Air Pressure Sensor 'A'' Circuit Low
Defective internal output driver or reference circuit in the control module for Reductant Injection Air Pressure Sensor 'A'' Circuit Low
Symptoms
Recommended Solution
Compare signal voltage and resistance between circuit A and companion circuit B at module connector.
Measure signal wire resistance to chassis ground with connector unplugged (must exceed 10 kΩ; low reading confirms short to ground).
Verify loaded supply voltage and harness terminal pins at Reductant Injection Air Pressure Sensor 'A'' Circuit Low.
Check circuit continuity and ground-side voltage drop (< 0.05 V) for Reductant Injection Air Pressure Sensor 'A'' Circuit Low.
Check supply and the low-side driver at the ECM connector. Replace the actuator or repair the circuit based on the readings.
Mechanic's Tip
Use the scan-tool output test with the scope on the control side at the same time. If duty changes but the actuator does not move, the valve is stuck — not the electrical fault you were chasing.