Mercedes Sprinter SCR Efficiency Fault — Full Investigation
2018 Mercedes Sprinter 314 CDI (OM651, Euro 6)· St Albans, Hertfordshire
A courier Sprinter on its third NOx sensor in 14 months — but the real fault was upstream. Here is how we traced it.
Reported symptoms
- 'Check AdBlue — see operator's manual' on cluster
- EML on, with intermittent limp mode under load
- P20EE returning every 200–400 miles after each clear
- Two replacement sensors fitted by a previous garage with no lasting fix
Diagnostic process
- Xentry full system scan — confirmed P20EE active, P2200 in history
- Live data on both NOx sensors — upstream reading low at higher loads
- Smoke-tested exhaust upstream of SCR — leak found at flexi joint
- Inspected dosing injector spray pattern — within spec
- Checked AdBlue quality — fresh, in spec
Findings
An exhaust leak upstream of the SCR was causing the upstream NOx reading to fall, which in turn made SCR efficiency calculations appear to fail. Every replacement NOx sensor had been fitted to a system that was still leaking — so each one logged the same code within a few hundred miles.
Resolution
- Repaired upstream exhaust flexi joint (no replacement sensor needed)
- Pressure-checked the system post-repair
- Reset SCR adaptation values and cleared stored codes
- Road-tested under load to confirm both NOx sensors reporting in spec
Outcome
Saved the operator a fourth unnecessary NOx sensor replacement. Vehicle returned to service the same day; no recurrence at follow-up 3,000 miles later.
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