You can technically drive with a faulty DEF (Diesel Exhaust Fluid) sensor—for a limited time—but it comes with serious restrictions and consequences. Most modern diesel engines equipped with SCR (Selective Catalytic Reduction) systems are programmed to comply with emissions regulations. When the DEF sensor malfunctions, the system assumes emissions control is compromised and will respond accordingly.
What Happens When the DEF Sensor FailsWhen the system detects a DEF sensor issue, it usually triggers:
- A check engine light or emissions system warning
- Diagnostic Trouble Codes (DTCs), such as P203F or P206A
- An initial grace period, allowing limited driving before further action is taken
If the fault isn’t resolved, the engine will initiate an emissions derate, which may include:
- Speed restrictions
- (e.g., limited to 5 mph)
- Idle-only operation
- in severe cases
- Prevention of engine restart
- until the issue is fixed
The countdown to these restrictions varies by manufacturer and model, but typically ranges from 100 to 300 miles once the fault is detected.
Legal and Compliance RisksDriving with a faulty DEF sensor may place your vehicle out of compliance with EPA regulations. This can be a significant liability for commercial fleets or anyone operating in regulated emissions zones.
What You Should Do- Check codes immediately
- using a diagnostic scanner
- Do not ignore warning lights
- or messages related to the emissions system
- Schedule service or replacement
- of the sensor or DEF module as soon as possible
In some cases, especially with older units, the sensor is integrated with the DEF tank. This means the entire tank may need to be replaced, which can be costly and time-sensitive.
Bottom LineYes, you can drive for a short time with a bad DEF sensor—but it's not recommended. Addressing the issue early helps prevent being stranded, fined, or stuck in limp mode.