P20E8 - Reductant Pressure Too Low
System
Powertrain (Engine & Gearbox)
Technical Description
The control module has detected a fault in the Reductant Pressure Too Low (P20E8). 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 Pressure Too Low – Coil Resistance | Resistance (Ω) | 3 - 30 Ω (typical) | Connector disconnected at 20°C |
| Reductant Pressure Too Low – Duty Cycle | PWM (%) | 5 - 95 % (modulates with target) | Engine running, actuator in control |
| Reductant Pressure Too 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 Pressure Too Low
Chafed or pinched wiring harness along the signal path to Reductant Pressure Too Low
Internal mechanical wear or carbon deposits inside Reductant Pressure Too Low
Defective internal output driver or reference circuit in the control module for Reductant Pressure Too Low
Symptoms
4
Recommended Solution
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 Pressure Too Low.
Check circuit continuity and ground-side voltage drop (< 0.05 V) for Reductant Pressure Too 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.