P0400 is a standard code that indicates your Mazda Miata’s EGR system is not working. The good news is that this code is not a breakdown risk and should not affect how your Miata runs (assuming it’s the ONLY code).
P0400 is a generic code, which means it has the same definition for the Miata as any other vehicle.
It is most commonly caused by a bad EGR valve, wiring, or vacuum line.
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.
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 Miata’s PCM.
P0400 Symptoms: Mazda Miata
There aren’t usually any noticeable symptoms with P0400 in the Miata. 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.
Mazda Miata P0400 Causes + Diagnosis
Here are the most common causes of P0400 in the Miata 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 Miata’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 Miata’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 Mazda Miata is most likely caused by a bad vacuum line, wiring issue, or EGR valve.
If there is anything you’d add that could help the next person who reads this article, please leave a comment below.