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P0400 Code – What Does It Mean & How To Fix It

This is one of the most frequent OBD2 trouble codes. Read the full article below to know what it means, how to fix it, and what other codes may show related to it.


This code represents an exhaust gas recirculation flow malfunction.Every car has a system that controls its emissions. In this system, a certain amount of exhaust gases are directed back into the engine. This process lowers the combustion temperature, which in turn allows the engine to run more smoothly and efficiently. You’ll see the code P0400 when the system registers that this flow of exhaust gases is lower than expected.


P0400  is an OBD-II generic code. It means that the engine control module (ECM) found a problem with the engine exhaust gas recirculation (EGR) valve.

In this case, the valve is not flowing the proper level of recirculated exhaust gases when it is being told. If it was working properly, the valve would open the flow of gases into the intake manifold.


There are a few reasons behind why this code could appear. It could be that the EGR valve is failing to open. This is usually due to a lack of vacuum, which would otherwise open the valve from the EGR control solenoid. It could also be that the EGR simply is not holding the vacuum.

Another reason could be that the EGR temperature sensor can’t register a temperature change. When the valve opens, the exhaust gas should flow through, and lower the combustion temperature.

However, if the sensor can’t pick up the change, P0400 could occur. A third reason why this problem happens comes from blockages in EGR passages. Carbon deposits, called soot, can clog up the EGR system passages and tubes.

This buildup will prevent the flow of exhaust gases into the intake manifold if left untreated over time.


The symptoms of this problem are fairly straightforward. The check engine alert will light up, and the code will be set in ECM memory. There could be the presence of an ignition ping, or a knock on acceleration.

Most crucially, the engine will not pass an emissions test, due to excessive amounts of NOx in the exhaust.


To verify the problem, scan the codes and documents in the freeze frame data. Then, clear the engine and ETC codes, and road test the car, checking to see if the code returns.

Visually inspect the vacuum system, including the hoses, wiring, and connections to the EGR valve and control solenoid. Disconnect and check the EGR valve. See if it is getting vacuum when the control solenoid opens when accelerating lightly and moderately.  

Check the EGR temperature sensor, as well. Look for a change, and observe if the engine bogs down when the EGR is open.

Lastly, remove the EGR valve and the temperature sensor and inspect for excessive soot buildup, or some other type of blockage.

Common mistakes

There are a few ways in which correcting a P0400 code can go wrong. One way is by replacing the EGR valve before testing the temperature sensor. If there is still carbon build up on the sensor, replacing the valve will do nothing to alleviate the problem.

Similarly, if the valve is replaced before checking the EGR pressure transducer, the P0400 issue may still exist.

When a malfunctioning pressure transducer is at the heart of the problem, replacing the valve will not fix it.

How serious is it?

P0400 is an emissions issue, and not a small one. If the code appears due to a bad EGR valve, failing to fix it can cause internal engine damage. The piston and valves are particularly vulnerable in this case.

Furthermore, the check engine light results in a failed emissions test. Excessive NOx gases are exactly what the emissions test is trying to prevent, so this code can be the difference between the car being road ready or not.

What repairs can fix the code?

repair manuals

Fortunately, there are a few obvious repairs which can fix P0400, provided they are at the heart of the problem. Replacing the EGR valve is an obvious one.

Another is to replace the broken vacuum line to the EGR valve. If the temperature sensor is to blame, it may need to be replaced. If it doesn’t need to be replaced, it may just need to be cleaned.

Remove the carbon build up in order to repair it. The EGR tubes that lead to the intake manifold may need to be cleaned, as well.

Related codes

Some related codes include P0401, P0402, P0403, and P0404.


The P0400 code is not something that can be ignored. If left untreated, it can cause serious damage to the vehicle’s engine.

Fortunately, there is an effective process to determine what the cause of the code is, and the solutions are clearly defined.

P0400 Code – What Does It Mean & How To Fix It