Short answer
SCR-equipped diesels use NOx sensors before and after the catalyst to judge how well the system is cleaning the exhaust. A failing sensor can turn on the check engine light, cause SCR efficiency warnings, change DEF use and lead to a derate. But an SCR efficiency code does not automatically mean a bad sensor: DEF quality, dosing and exhaust leaks can cause the same codes.
Start with the easy checks
- Read every engine and aftertreatment code, not just one.
- Inspect the sensor wiring and connectors for heat damage or corrosion.
- Look for exhaust leaks ahead of the sensors.
- Rule out DEF level and quality first.
Likely causes, most common first
- Failed sensor
- Sensors live in hot exhaust and can fail.
- Wiring and connector damage
- Heat, chafing and corrosion.
- Exhaust leaks
- Can skew readings.
- Real SCR or DEF problems
- The sensor may be reporting a true fault.
These are common causes, not a diagnosis. The same symptom can have a different cause on your engine; codes, tests and your service information decide.
When to stop and see a diesel tech
- Repeated SCR efficiency codes.
- Derates. A technician can compare sensor readings with live data before replacing parts.
Questions
Can a bad NOx sensor put a diesel in limp mode?
It can. On SCR-equipped engines, NOx sensor faults are among the faults that can lead to a staged derate.
Related symptoms
Why does my diesel say DEF fault or "speed limited"?
Troubleshoot →Power & drivabilityWhy did my diesel lose power or go into limp mode?
Troubleshoot →EmissionsWhat is DPF regeneration and why does my truck keep doing it?
Troubleshoot →Where this comes from
- HELLA Tech World: NOx sensor troubleshooting
- Diesel Aftertreatment: NOx sensor function, symptoms and testing
- Volvo SCR troubleshooting guide filed with NHTSA (PDF)
Last reviewed 2026-10-19. General information only: confirm with your service information and a qualified diesel technician.