SCR system mainly includes urea water tank, conveying device, metering device, injection device, catalytic converter, temperature and exhaust sensor, etc. The function of urea nozzle as a spraying device is to spray urea solution according to the instructions of the controller. Due to the influence of many factors, urea nozzles are prone to blockage and crystallization. Once the urea nozzle is crystallized, the nozzle will completely lose the spray effect. After the ECU computer control system is identified, it will alarm, restrict the engine or cause damage to the gas pump. If the injection system parts are blocked, the relevant assembly parts need to be replaced. The cost of replacing SCR system parts will increase the cost of the owner.
2. Causes of blockage and damage of urea nozzles:
1. The quality of urea solution for vehicle is not up to the standard.
Too many incompatibles and impurities in low-cost and inferior urea solution for automobiles on the market will cause blockage and damage of nozzles.
2. Residual solution after injection of urea solution nozzle
After the ignition of the engine is closed, the residual urea in the nozzle pipeline will crystallize and cause the pipeline and nozzle blockage.
3. SCR system does not work for a long time
The urea nozzle is located in the exhaust pipe and is in the high temperature environment for a long time. When it works, it is cooled by the circulation of urea solution. If the cooling effect of urea solution is not obtained, the urea nozzle is easy to be damaged. The long-term non-operation of SCR system will lead to the crystallization of residual urea solution in SCR system and clogging of pipes and nozzles, and the long-term lack of liquid immersion will reduce or even damage the life of parts of SCR system.
4. OBJECTIVE FACTORS
Complex physical and chemical reactions occur in SCR system, including atomization, fragmentation, evaporation of urea solution, energy and momentum exchange between droplets and exhaust gas, particle impingement process, liquid film formation, catalytic reduction reaction of Nox, etc. The atomization field and temperature field formed after urea injection change with the change of engine operating conditions. Urea droplets are decomposed into ammonia gas. It also produces intermediate products such as cyanic acid, biuret and cyanuric acid, which are easy to form urea crystals and other sediments. Sediments accumulate and can cause nozzle blockage.
These factors will cause the blockage of urea nozzles, so that the majority of drivers can not be avoided.
3. Common Faults of SCR System and Solutions
The urea box related faults mainly include the shell of the urea box and the liquid level temperature sensor assembly.
The common faults of the sensor are inaccurate liquid level display, abnormal temperature display, and the faulty lamp is always on and the faults of the liquid level temperature sensor are reported.
The main causes of these faults are: sensor damage, virtual connection of sensor connectors, short circuit, and related wiring harness faults.
Sometimes, the mismatch between the liquid level temperature sensor and the electrical parameters of the main engine factory will cause the abnormal liquid level and temperature, and even report the fault of the sensor.
1. Urea level sensor voltage higher than upper limit (lower limit) - sensor connector needle withdrawal
Fault phenomena:
Failure lamp and OBD lamp are always on, flashing code 445 (urea level sensor voltage is higher than the upper limit) is reported, and the display of urea level in the instrument is inaccurate.
Fault mechanism:
This fault is usually caused by the open circuit of the sensor harness or connector. Please check whether the sensor connector No. 1 pin (ECU pin K57) has open circuit and short circuit with the power supply. If it can't be solved, check the other pins and wiring harness for faults.
Possible reasons:
1) The connector of urea level sensor or the large plug of the whole vehicle wiring harness withdraws the needle;
2) Open or virtual connection of urea level wiring harness;
3) short circuit between K57 and power supply;
4) damage of urea level sensor.
Practical solutions and steps:
1) Check the connector and wiring harness of urea level sensor;
2) Pull out the connector of urea level sensor to check whether there is needle withdrawal causing open wiring harness.
3) If yes, re-fix the pin.
2. Abnormal Display of Urea Level and Temperature
Fault phenomena:
Urea liquid level display is inaccurate (for example, when urea is very small, the instrument shows urea 100%). Urea temperature is very different from the current ambient temperature, and there is no report of sensor failure.
Fault mechanism:
This kind of fault is usually due to the mismatch between the urea box sensor and the designation of the main engine factory, or the replacement of the urea box in the whole vehicle, which makes the sensor inside the urea box different from the original model before leaving the factory. The different electrical parameters of the sensor result in mismatch of data calibration and wrong display of liquid level temperature.
Possible reasons:
1) The driver replaced the urea box, which is different from the original urea box.
2) When matching the whole vehicle factory, the urea box is purchased independently, but the technical personnel of the main engine factory are not informed to re-calibrate the data.
3) The sensor or related wire harness is damaged, which results in the change of electrical parameters (which is less likely), but no sensor-related faults are reported.
Practical solutions and steps:
1) Check the urea box and sensors to verify whether they are the same as the original urea box.
2) There are two main faults of urea pump: mechanical faults and electrical faults. Electrical faults generally refer to the faults of electrical components related to 12-hole connectors, including the motor of urea pump, urea pressure sensor, reversing valve and heating resistance wire of urea pump, with relatively high failure rate; mechanical faults refer to the blockage of urea pump, the failure of building pressure caused by the failure of mechanical parts inside urea pump, etc.
In a word, the malfunction related to SCR urea pressure should first check whether the urea pipeline is misconnected, blocked and leaked, and whether the urea pump is blocked.
3. SCR Urea Pressure Establishment Error - Mechanical Fault Code of Urea Pump:
441:SCR Urea Pressure Establishment Error.
Fault phenomena:
After several minutes to tens of minutes of each sports car, failure lights and OBD lights are always on, and 441 (SCR urea pressure build-up error) faults will be reported, urea does not consume.
Fault mechanism:
Before urea injection, urea pump builds up urea to 9 bar pressure, which is detected by pressure sensor in urea pump. When the engine starts up, the urea pump tries to build pressure on urea several times. If the urea pressure is still less than 9 bar, the malfunction will be reported.