14/08/2026 6:12 am
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Automotive Q&A
NOx plausibility code appeared immediately after SCR exhaust service
Review the evidence, add a focused answer, and distinguish confirmed measurements from assumptions.
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A 2019 Sprinter sets a NOx plausibility code after the SCR assembly was removed for service. Both sensors report ambient-like values cold, but once hot the downstream value occasionally freezes while the upstream value changes with load. Sensor part numbers are correct and no dosing fault is present.
Before replacing the downstream sensor, what installation and data checks matter most?
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15/08/2026 8:42 am
Confirm each connector reaches the correct upstream or downstream sensor and that no extension lead is twisted, overheated or left under tension. Check sensor control-unit power, ground and network communication while the value freezes. Compare heater status, dew-point release and actual NOx response during a controlled road test; a sensor that is not yet enabled should not be judged as frozen.
Verify any required coding or adaptation after service. Because the fault began immediately after disassembly, connector locking and harness routing deserve inspection before component replacement.
Topic starter
16/08/2026 7:45 am
The downstream connector shell looked latched, but its secondary lock had stopped short and one communication terminal backed out under vibration. Reseating the terminal restored continuous data. Upstream and downstream readings then changed plausibly through an SCR efficiency test.