P0400 Exhaust Gas Recirculation Flow Malfunction

OBD-II Trouble Code - P0400 - Data Sheet

P0400 - Malfunction of the exhaust gas recirculation (EGR) system

What does trouble code P0400 mean?

This Diagnostic Trouble Code (DTC) is a generic transmission code, which means it applies to OBD-II equipped vehicles. Although general, specific repair steps may differ depending on the brand / model.

The Exhaust Gas Recirculation (EGR) valve is a vacuum operated valve that regulates the amount of exhaust gas re-entering the cylinders.

The powertrain control module (PCM) determines the value based on engine load, temperature, and other conditions. If the PCM detects that the amount of exhaust gas entering the cylinder was insufficient or absent, this code is set.

Symptoms

The driver will most likely not notice any other symptoms other than the MIL (Malfunction Indicator Light). However, subtle symptoms will be an increase in combustion temperature and an increase in NOx emissions.

• Turn on the engine warning light on the dashboard.
• Increased NOx emissions as well as increased combustion temperature.
• Possible vibrations of the engine.

Causes of the P0400 code

The P0400 code may mean that one or more of the following events have occurred:

• Clogged exhaust gas recirculation duct, which restricts the flow of exhaust gases.
• Exhaust gas recirculation solenoid defective
• Defective exhaust gas recirculation solenoid valve wiring / harness
• Vacuum lines damaged / disconnected from the EGR valve solenoid or from the EGR valve.
• Exhaust gas recirculation valve defective
• Damaged or defective EGR valve. The EGR valve may be stuck or closed.
• Faulty or damaged EGR temperature sensor and circuits.
• Open or short in the EGR valve wiring harness.
• Poor electrical connection to the EGR valve.
• The EGR passage is blocked, restricting the flow of exhaust gases.
• Damaged or disconnected vacuum hoses from the EGR valve solenoid.

Possible solutions

Since the designs of the exhaust gas recirculation valve differ, a test alone will not be enough:

• Using a scan tool, operate the EGR valve with the engine running. If the engine stumbles, the problem was most likely an intermittent wiring failure or intermittent blockage.
• If the engine does not stumble, manually operate the EGR valve if possible. Unless the engine trips or stalls, the ports are likely clogged. Valve removal and cleaning of all ports will be required.
• The solenoid test can usually only be performed with a scan tool because most solenoids operate with a voltage duty cycle rather than a constant voltage.
• Check all vacuum lines, hoses, etc. for damage.
• Inspect the solenoid harness and solenoid for damage.
• Replace the exhaust gas recirculation valve.

Associated EGR Codes: P0401, P0402, P0403, P0404, P0405, P0406, P0407, P0408, P0409

Common Mistakes When Diagnosing Code P0400

• Replacing the EGR valve before checking for carbon deposits on the EGR temperature sensor.
• Replacing the EGR valve without checking the EGR pressure sensor.

How serious is the P0400 code?

• A faulty EGR valve can cause the engine to over-ignite, which can cause internal damage to the engine piston and valves.
• A lit Check Engine light will cause the vehicle to fail the emissions test due to excessive NOx.

What repairs can fix code P0400?

• Replacing the EGR valve
• Replacing a broken vacuum line to the EGR valve
• Replacing the EGR temperature sensor or cleaning it from soot to repair it
• Removing carbon deposits from the EGR pipes to the intake manifold

Additional comments to consider regarding code P0400

Code P0400 is triggered when the EGR temperature sensor does not see a change in temperature when the EGR is commanded to open. These sensors tend to accumulate a lot of carbon, which causes them to become insensitive to heat from EGR gases.

How to Fix P0400 Engine Code in 3 Minutes [2 DIY Methods / Only $4.11]

Need more help with your p0400 code?

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