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2018 RAM 3500
6.7L I6 Cummins•RWD•diesel•ISB•turbo
11 active safety recalls on this vehicle — view recalls
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Repairs0Labor515Torque136Fluid9DTC1795Battery1Maintenance0Recalls11
P3838moderate severityPowertrain

NOx Sensor Downstream of SCR Rationality Error

on 2018 Ram 3500 6.7L I6 Cummins · RWD

What this code means

The downstream NOx sensor reading does not correlate with expected values based on DEF dosing rate and upstream NOx levels.

Symptoms

  • Check Engine Light
  • Incorrect SCR efficiency calculation
  • False derate warnings possible
  • DEF dosing may be incorrect
  • Emissions test may show inconsistent results

Common causes

  • NOx sensor drift or failure
  • Sensor cross-sensitivity to ammonia
  • Sensor wiring intermittent
  • Exhaust leak near sensor
  • Sensor poisoned by contaminants

Diagnostic steps

  1. Compare upstream and downstream NOx sensor readings
  2. Check sensor readings at key operating points
  3. Test sensor with known gas concentration
  4. Inspect sensor wiring and connector
  5. Check for exhaust leaks near sensor

Repair notes

Replace downstream NOx sensor. Verify SCR efficiency calculation normalizes after replacement. Use OEM sensor only.

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