How to troubleshoot low SCR conversion efficiency (SCR actual efficiency deviation exceeding threshold ) (SCR diagnostic efficiency) faults
When the diagnostic tool reports a fault code such as low SCR conversion efficiency (SCR actual efficiency deviation exceeding threshold 1) (SCR diagnostic efficiency 1), Prove that there is an abnormality in the monitoring of SCR conversion efficiency after the closed-loop operation of the nitrogen and oxygen sensors before and after urea injection for a period of time. If the average actual conversion efficiency of nitrogen and oxygen is lower than the average conversion efficiency limit corresponding to NOx emissions of 1.2g/Wh under relatively stable conditions, the system will judge that the actual NOx emissions exceed the emission limit of 1.2g/Wh. Based on this, activate the corresponding failure strategy, including the driver induction system. At this time, the fault on the graph will be reported, The fault phenomenon shows that the OBD fault light is on, and the vehicle is limited in torque and speed.
Common reasons:
1. Measurement deviation of nitrogen and oxygen sensors: low upstream nitrogen and oxygen measurement value/high downstream nitrogen and oxygen measurement value)
2. The urea concentration does not meet the technical specifications: the urea concentration is too low
3. Relevant failures of urea nozzle: regular spraying, poor atomization, liquid leakage, nozzle hole blockage
4. Urea filter blockage fault: urea injection pressure decreases, urea injection flow rate decreases
5. Diesel sulfur content exceeding standard, urea crystallization blockage, SCR coating poisoning)
When repairing such faults, connect the diagnostic tool to first read the current/historical fault codes, and focus on the relevant fault codes of the urea injection system, nitrogen oxygen sensor, etc. If there are faults that can cause abnormal or impossible injection of the urea system, priority should be given to troubleshooting the urea system fault or other fault codes that can cause excessive emissions.
For example: NOx sensor malfunction, exhaust temperature sensor malfunction, urea pump malfunction, urea nozzle malfunction, urea quality malfunction, EGR system malfunction, turbocharger malfunction, urea heating system malfunction, etc.