Diagnostic Trouble Code P0400

P0400 “Exhaust Gas Recirculation Flow Malfunction”


What is code P0400?

Code P0400 stands for “Exhaust Gas Recirculation Flow Malfunction”. This diagnostic trouble code can show up for reasons related to the exhaust gas recirculation, or “EGR” system on your vehicle. Therefore, it is important to have a qualified technician diagnose the specific problem that caused the code to be stored in your vehicle’s computer.

Get this checked out soon by a CarAdvise shop, where the work will be guaranteed, and one of our expert techs will review any repair orders.

What does code P0400 mean?

If a diagnostic scan of your vehicle indicates a code P0400, a technician knows that there is something going on with your exhaust gas recirculation (EGR) system.

The EGR system helps your engine to run efficiently, consume less fuel, and emit fewer emissions. The EGR accomplishes these things by recirculating specific amounts of exhaust gas back to the intake system and ultimately to the combustion chambers. This recirculation of the exhaust gases changes the chemical composition of the air that enters the engine, decreasing the amount of oxygen so that combustion temperatures are minimized. In the process, less of the hazardous nitrogen oxide gas is produced as a byproduct of combustion. This allows for a cleaner and more efficient exhaust system. EGR systems are controlled either electronically or by vacuum, depending on your vehicle make, model, and age

If your vehicle’s ECM (Engine Control Module) detects that the exhaust gases from the EGR system are not flowing properly when the EGR valve is open, it will store code P0400 and turn on the check engine light..


What causes code P0400?

A number of causes can trigger a code P0400. That is why an accurate diagnosis is essential. Causes of this code include:

Most likely

  • Faulty or plugged EGR valve ($200-$500)
  • Vacuum leak to the EGR valve
  • Carbon-plugged EGR tube (Avg. $250)

Other possibilities


What symptoms are associated with this code?

Common signs associated with a code P0400 include the following:

  • The check engine light is on or flashing
  • Reduced fuel economy
  • Spark knock from the engine under a load
  • Engine runs roughly and might be hard to start


How does a technician diagnose code P0400?

If a diagnostic scan of your vehicle indicates a code P0400, a technician will perform tests to pinpoint the problem. These tests include checking for any related trouble codes. The technician will also visually inspect the EGR system and look for any loose or cracked vacuum lines in the engine compartment. A multimeter might be used to run tests on the EGR valve, and the technician might manually open the valve while the engine is running to see if there are any changes to engine speed. If necessary, the technician might need to remove the EGR valve, cooler, and tubes to clean or inspect.


How serious is this code?

In many cases, a code P0400 will not cause any major drivability problems. On the other hand, issues with the EGR system can cause a drop in fuel economy, which will show up over time. Engine spark knock, if left untreated, can cause serious damage to internal components, such as the pistons and valves. While this code is not necessarily a cause for immediate concern, it should not be ignored. Therefore, it is recommended that you have your vehicle inspected and serviced as soon as it is reasonable to do so, typically within one to two weeks.

Get this checked out soon by a CarAdvise shop, where the work will be guaranteed, and one of our expert techs will review any repair orders.


Related Diagnostic Trouble Codes

P0401 Exhaust Gas Recirculation Flow Insufficient Detected

P0402 Exhaust Gas Recirculation Flow Excessive Detected

P0403 Exhaust Gas Recirculation Control Circuit

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