Hyundai Equus P0400: Meaning, Causes, + Diagnosis |

Drivetrain Resource

P0400 Hyundai Equus

P0400 is a standard code that indicates your Hyundai Equus’s EGR system is not working. The good news is that this code is not a breakdown risk and should not affect how your Equus runs (assuming it’s the ONLY code).

P0400 is a generic code, which means it has the same definition for the Equus as any other vehicle.

It is most commonly caused by a bad EGR valve, wiring, or vacuum line.

P0400 Definition

P0400 Meaning: Hyundai Equus

Here’s the definition of P0400 broken down into two parts. Understanding why you got the code is critical to diagnosing and fixing it.


The exhaust gas recirculation system (commonly referred to as the EGR system) allows exhaust gas to enter the combustion chamber a second time. The EGR system provides no engine performance benefit; this is done purely for emissions compliance.

In fact, it’s one of the oldest forms of emissions technology there is, which means that there has been a lot of time to refine it.

Flow Malfunction

Exhaust gasses are not flowing into the combustion chamber, which indicates that the EGR valve/solenoid is not working correctly. There are a few common reasons for this issue (discussed in depth below).

When the exhaust gasses are not flowing into the combustion chamber properly, P0400 will be stored in your Equus’s PCM.

P0400 Symptoms: Hyundai Equus

There aren’t usually any noticeable symptoms with P0400 in the Equus. The most common ones are:

  • Check engine light
  • Increased engine emissions
  • Slight increase in combustion temperature

This code is a common reason why vehicles fail an emissions inspection.

Hyundai Equus P0400 Causes + Diagnosis

P0400 Diagnosis

Here are the most common causes of P0400 in the Equus and an excellent diagnostic order to use. You’ll need a good scan tool to command the EGR solenoid to open or close. We’ll cover what you can do without one first.

Check the EGR System Wiring

Check the wiring going to the EGR valve and solenoid for any apparent signs of damage. Look for burnt, cracked, or broken wires. If there are, you’ll need to repair the harness.

Examine the harness where it plugs into the EGR valve. Ensure that it is plugged in tight and that the pins aren’t damaged or corroded.

Check the Vaccum Lines

There are two main types of EGR valves, vacuum controlled and electronically controlled. Most newer motors use the electronically controlled type.

Depending on your Equus’s model year and engine type, it could have either.

If vacuum lines are going to the EGR, make sure that they are not cracked or leaking.

Test the EGR Valve With a Scan Tool

You can command your Equus’s EGR valve to open and close with a good scan tool. When you do so, there should be some reaction from the engine. It should at least momentarily stumble and maybe even die.

Check out the YouTube video below. It’s 3 minutes long and gives an excellent idea of how to test the EGR system with a scan tool.


P0400 in the Hyundai Equus is most likely caused by a bad vacuum line, wiring issue, or EGR valve.

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