Mercedes W221 S-Class NOx Sensor Failure — AdBlue Warning
Mercedes-Benz
S-Class
2007
OM642 BlueTEC
⚡ Quick Summary
Severity Level
Medium
Safe to Drive?
⚠️ Drive with Caution
Safety Impact
Indirect risk — worth monitoring, not urgent
Urgency Level
🟡 Monitor Closely
The NOx sensor on W221 S350 BlueTEC and S320 CDI models fails, triggering AdBlue warning lights and ultimately an engine start countdown if not resolved. The sensor reads incorrect NOx levels and causes the SCR system to inject incorrect AdBlue quantities. Sensor replacement costs £200-450.
Repair / Solution
No confirmed fixes yet. Share yours below.
⚠️ Wrong Diagnosis?
No commonly reported misdiagnoses for this fault yet. If you have replaced a part that did not fix this Mercedes-Benz S-Class 2007 problem, submit a wrong diagnosis report — it helps other drivers avoid the same costly mistake.
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