P20EE - SCR NOx Catalyst Efficiency Below Threshold (Bank 1)
System
Powertrain (Engine & Gearbox)
Technical Description
The control module has detected a fault in the SCR NOx Catalyst Efficiency Below Threshold (Bank 1) (P20EE). Affects cylinder bank 1 (bank 1 containing cylinder 1, firewall/right side).
Multimeter Test Points & Nominal Values
| Test Point / Component | Measurement | Nominal Target Range | Test Condition |
|---|---|---|---|
| SCR NOx Catalyst Efficiency Below Threshold (Bank 1) – Coil Resistance | Resistance (Ω) | 3 - 30 Ω (typical) | Connector disconnected at 20°C |
| SCR NOx Catalyst Efficiency Below Threshold (Bank 1) – Duty Cycle | PWM (%) | 5 - 95 % (modulates with target) | Engine running, actuator in control |
| SCR NOx Catalyst Efficiency Below Threshold (Bank 1) – 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
Bank 1 specific:Inspect wiring harness to cylinder bank 1 (firewall/right bank, exposed to exhaust manifold heat).
Chafed or pinched wiring harness along the signal path to SCR NOx Catalyst Efficiency Below Threshold (Bank 1)
Internal mechanical wear or carbon deposits inside SCR NOx Catalyst Efficiency Below Threshold (Bank 1)
Defective internal output driver or reference circuit in the control module for SCR NOx Catalyst Efficiency Below Threshold (Bank 1)
Symptoms
4
Recommended Solution
Cross-swap the component on Bank 1 with Bank 2 to rule out mechanical oil passages and internal timing wear.
Review freeze-frame environmental data and perform fundamental voltage-drop and resistance checks on the circuit.
Verify loaded supply voltage and harness terminal pins at SCR NOx Catalyst Efficiency Below Threshold (Bank 1).
Check circuit continuity and ground-side voltage drop (< 0.05 V) for SCR NOx Catalyst Efficiency Below Threshold (Bank 1).
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.
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