When a DEF sensor malfunctions, it can trigger several issues in your vehicle's emissions system. Here's what typically happens:

‌1. Warning Lights and Error Codes‌
The DEF sensor monitors the quality and level of diesel exhaust fluid. If it fails, the engine control unit (ECU) will detect the fault and illuminate the ‌check engine light‌ or a specific ‌DEF warning light‌ on the dashboard. It will also store a diagnostic trouble code (DTC) in the ECU memory, such as ‌P204F‌ (DEF quality sensor circuit) or ‌P204E‌ (DEF quality sensor performance).

‌2. Reduced Engine Performance‌
To prevent damage to the emissions system, the ECU may enter a ‌"limp mode"‌ or ‌"reduced power mode"‌. This limits engine power and torque to ensure the vehicle can still be driven safely to a repair shop.

‌3. Emissions System Failure‌
The DEF sensor is part of the selective catalytic reduction (SCR) system. A faulty sensor can cause the SCR system to malfunction, leading to increased emissions of nitrogen oxides (NOx). This can result in the vehicle failing emissions tests.

‌4. Potential Engine Shutdown‌
In severe cases, if the DEF sensor failure is not addressed, the ECU may shut down the engine to prevent further damage to the emissions system.

‌5. Increased Fuel Consumption‌
A malfunctioning DEF sensor can cause the engine to run less efficiently, leading to increased fuel consumption.

‌6. Costly Repairs‌
Ignoring a faulty DEF sensor can lead to more extensive damage to the emissions system, resulting in costly repairs.

‌7. Environmental Impact‌
Increased NOx emissions from a faulty DEF sensor can contribute to air pollution and have negative environmental impacts.

‌8. Legal Consequences‌
In some regions, driving a vehicle with a malfunctioning emissions system can result in fines or other legal consequences.

‌9. Diagnostic Challenges‌
Diagnosing a faulty DEF sensor can be complex and may require specialized diagnostic equipment and expertise.

‌10. Maintenance Costs‌
Replacing a faulty DEF sensor can be expensive, and the cost may vary depending on the vehicle make and model.

If you suspect a DEF sensor issue, it's best to have your vehicle inspected by a qualified technician as soon as possible.