Why does the fault code P0400 occur in diesel cars?
By
Vroomouk
This blog will explain the causes of the p0400 fault code and answer the following questions: how to fix the P0400 fault code? Can you drive the car with the P0400 fault code? What are the products available to clean the exhaust gas recirculation system?
Why does the fault code P0400 occur in diesel cars?
The fault code P0400 is related to a malfunction in diesel cars’ exhaust gas recirculation valve. It occurs due to the following reasons:
- Stuck exhaust gas recirculation Valve or actuator
- Malfunction in Exhaust gas temperature sensor
- Blockage in the intake air system
- Faulty hot film air mass sensor
All the reasons mentioned above are explained in detail below:
Stuck exhaust gas recirculation Valve or actuator
The EGR valve is located near the cylinder head of the engine. When the EGR valve opens the exhaust gases flow into the intake after passing through an EGR cooler. The EGR valve can get stuck due to excessive build-up of soot particles and the engine control unit cannot control the number of exhaust gases mixed in the fresh air.
Malfunction in Exhaust gas temperature sensor
The exhaust gas temperature sensor is present near the EGR valve. The temperature of the exhaust gases is measured by the sensor, if the temperature is too high the engine control unit will adjust the flow of the exhaust gases by actuating the EGR valve.
The EGR cooler bypass valve is also fully opened so the gases are cooled before entering the intake manifold. Hence when the exhaust gas temperature sensor malfunctions.
The engine control unit records the implausible temperature of the exhaust gases. It cannot control the amount of EGR entering the intake system and fault code P0400 is stored in the DTC memory.
Blockage in the intake air system
The intake system of the diesel engine consists of a throttle valve in the mixing chamber. The flow of fresh air is regulated in the mixing chamber so when the throttle valve gets blocked, only exhaust gases go into the intake manifold.
The flow of fresh air can also get blocked due to a choked intercooler hose. Hence it is necessary to inspect the intake as well as exhaust gas recirculation system for blockage caused due to excessive build-up of soot and carbon particles.
Faulty hot film air mass sensor
The engine control unit determines the mass of fresh air entering the intake system with the help of the air mass sensor. The signal from the air mass sensor also determines the number of exhaust gases entering the intake manifold.
A faulty air mass sensor will measure an incorrect amount of fresh air in the intake system and the flow of exhaust will be high. Black smoke comes from the exhaust and power is also reduced.
How to fix the P0400 fault code?
When the vehicle reports to the workshop with a fault code related to the exhaust gases recirculation valve, it is rectified by simply replacing the stuck EGR valve. As the exhaust gases flow through the valve into the EGR cooler, a lot of soot particles get accumulated on the valve.
The temperature of the exhaust gases is also high as the valve is located before the EGR cooler, the high temperature and soot particles eventually jam the opening and closing of the valve.
As the movement of the valve is restricted, it puts stress on the electronic actuator and the actuator also fails. Therefore, only cleaning the soot particle by removing the valve does not fix the problem. The complete EGR valve must be replaced to rectify the P0400 fault code.
If the actuation of the EGR valve is good then other components involved in the control system of the EGR are checked. Depending on the findings the exhaust gas temperature sensor or the hot air mass flow sensor can also need replacement if the EGR valve is already replaced.
Therefore, the actuation of the EGR valve is first checked at the workshop and based on the findings; the faulty components are renewed to rectify the fault code.
Can you drive the car with fault code P0400?
It is possible to drive the car with the fault code as diesel engines are quantity control engines. Air is always present in greater quantity and the amount of diesel fuel injected determines the power produced by the engine.
When the fault code is logged in the diagnostic trouble code memory of the engine control unit, the check engine light is activated in the instrument cluster. Based on the position of the stuck EGR valve the supply of fresh air can be reduced by up to 80 per cent.
Hence, driving a car with a stuck or faulty EGR valve can reduce performance. Black smoke and complete combustion also occur as the engine control unit continues to inject the same quantity of fuel during the compression stroke.
Driving the car with a check engine light ON can also lead to catastrophic engine damage as the driver is unaware of any other critical fault codes present in the engine control unit. Therefore, it is recommended to fix the P0400 fault code as the repairs do not cost a lot of money.
What are the products available to clean the exhaust gas recirculation system?
The products available to clean the exhaust gas recirculation system are listed in the table below. The exhaust gas recirculation system only requires cleaning when the mileage of the car is high.
| Product Name | Type | Link |
| CRC 05078 Throttle Body and Air-Intake Cleaner | Spray (invasive cleaning) | https://amzn.to/3bg3q0m |
| Liqui Moly 20208 Pro-Line Diesel Intake System Cleaner | Spray (non Invasive cleaning) | https://amzn.to/3OHFgth |
| Wynn’s 28263 Diesel Particulate Filter Cleaner | Additive (Mixed in Fuel) | https://amzn.to/3oBxGWj |
Invasive cleaning means parts of the exhaust gas recirculation system are disassembled and cleaned individually. It is more labour extensive but also the most effective type of cleaning.
Non-invasive cleaning involves spraying the cleaner into the intake system of the engine, ideally as close to the EGR valve as possible.
Fuel additives are the least effective type of cleaning method but the effect shows in the long run. Additives are mixed in the diesel fuel and it not only helps to keep the fuel system clean but also reduces the formation of soot particles in the exhaust gases.
Other articles about Vehicle Fault Codes you may be interested in
What are the symptoms of the P2422 fault code?
Why does the fault code P0400 occur in diesel cars?
What does the P500 fault code mean?
Conclusion
This blog explained the causes of the Fault code P0400 and also listed different types of products available to clean the exhaust gas recirculation system. EGR system is very crucial in diesel engines as it lowers the combustion temperatures and also reduced the formation of the NOX. Hence, the EGR system must always be fault free while operating the vehicle.
