Reductant System Performance Logic Fault

The computer in your vehicle isn't getting the right signals about how the exhaust treatment system is working, especially the fluid that's supposed to clean the exhaust.

The PCM sees data from the NOx sensor and the reductant injection system that doesn't match expected performance curves. It compares what the NOx sensor detects with how much reductant should be injected based on engine load and temperature.

Don't just throw parts at it. Walk through these in order — each step tells you whether to keep going or stop and fix what you found.

1. 1Check the DEF fluid level — it should be between min and max on the reservoir. (Decision: is the level adequate?)
2. 2Inspect the wiring and connectors to the NOx sensor and reductant system — look for signs of damage or corrosion. (Decision: are connections good?)
3. 3Use a scanner to check for additional fault codes related to the reductant system. (Decision: are there any other codes present?)
4. 4If wiring and fluid are good, perform a NOx sensor functionality test using the scanner while monitoring live data to see if readings are within expected range. (Decision: does the sensor read correctly?)
5. 5Test the operation of the reductant pump and check for clogs in the injector if the NOx readings are incorrect. (Decision: is the pump and injector functioning properly?)

• Faulty NOx sensor
• Low DEF fluid level
• Clogged DEF injector
• Faulty reductant pump
• Wiring issues in DEF circuit

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