Complete List of Common System Faults and Troubleshooting Methods for SCR

The working principle of the SCR system is that under the action of a catalyst, ammonia (NH3) preferentially reacts with nitrogen oxides (NOX) in the exhaust gas to generate nitrogen and water, rather than reacting with oxygen. Ammonia (NH3) is a colorless gas with a strong pungent odor, which is inconvenient to store and transport. In engineering practice, vehicles store reducing agents as solutions of urea and water. Urea is sprayed into the exhaust pipe under pressure, and atomized urea decomposes when heated, generating ammonia gas and participating in the reaction.

1. SCR system disconnection fault

Symptoms: The engine is diagnosed as disconnected from the SCR system, the MIL fault indicator light is on, and the urea instrument level is not displayed.

Troubleshooting method: Firstly, check whether the data transmission between the SCR and the engine is normal, that is, check whether the SCR is working properly through a diagnostic tool. If it is normal but the engine speed information cannot be received, check whether the connection bus (CAN bus) between the SCR and the engine is normal, whether there is a short circuit or open circuit, and whether the high and low voltages of the CAN bus are reversed; Recheck if the SCR system is powered on. Check if the pins and voltage of the vehicle’s SCR power supply are normal. If they are not normal, check the vehicle’s wiring. If they are normal, check if the fuse is burned out and if the relay is normal; Finally, check if the DCU is faulty. If the diagnostic tool cannot connect to the system, check if the DCU is powered on normally. If it is normal, it may be due to a DCU malfunction and needs to be replaced.

2. NOx sensor malfunction

Fault phenomenon: The fault light is on, and the diagnostic tool displays NOx. Sensor malfunction.

Troubleshooting: Check for NOx. Sensor heating: After power on, gently touch the sensor. If there is no heating, check if the 24V power supply is normal. If there is no voltage, check if the circuit is short circuited or open: if there is voltage, it may be a sensor fault that needs to be replaced. Check if the wiring of the NOx sensor connection is normal. If the fault is still not resolved, replace the NOx sensor. If the fault is resolved, it is confirmed that the NOx sensor is faulty.

Bosch airless SCR system