P0605 is a relatively rare OBD-II trouble code that can occur in the Mazda Miata. It indicates that PCM (powertrain control module) is not passing its ROM (read-only memory) self-test when you turn the ignition on.
While P0605 is a generic code (it has the same meaning for any car or truck), it is most frequently seen in Ford and Nissan-made vehicles.
The most common cause of P0605 is a bad PCM. If you’ve recently had your Miata’s PCM reflashed and you ended up with P0605, you’ll need to have them repair or replace it.
P0605 Definition: Internal Control Module – ROM Error
Your Miata’s PCM uses Read-Only Memory (ROM) to store its operating system, just like your computer keeps its operating system on a hard disk.
Every time you start your engine, the PCM tests its ROM by running the same mathematical calculations. Since ROM is meant to be read and not written to, the value returned from the equation should always be the same number. When it’s not, P0605 is stored, and the check engine light comes on.
Mazda Miata P0605 Symptoms
Without fail, there are almost always noticeable symptoms that accompany P0605. Here are the most common ones:
- Transmission may shift erratically
- Limp mode
- Misfiring (you’ll most likely have P0300, random misfire)
- No start
- Dash warning lights
Even if your Miata’s engine appears to be running fine, we don’t recommend driving with P0605. The code indicates that there is a serious issue with the PCM. Modern vehicles can’t run or drive at all if the PCM fails.
Mazda Miata P0605 Causes and How to Fix
A bad PCM almost always causes P0605, but you should check your Mazda Miata’s wiring harness before sending it in for repair.
Bad Wiring Harness
Check the ground to the PCM. You’ll need a printout of the circuit for your particular engine and model year. Make sure that it’s not damaged or corroded. Check that the voltage level going into the PCM is correct. If it’s off, P0605 can be triggered.
Unfortunately, replacing the PCM is the most common way to fix P0605. However, replacing it is not as simple as buying a new one online. You need one that is matched to your Miata’s engine, transmission, model year, VIN, security system, and more.
A good diagnostic shop (or the dealer) can flash the new PCM and get it right. They can test the wiring. Sometimes, they even are able to save the old PCM.
If you want to try replacing the PCM at home, there are websites online, such as Car Computer Exchange (not an affiliate link), where they can tune it for you based on your VIN. You’ll have to mail in your old PCM.
P0605 is most likely caused by a bad PCM. If you’ve recently had your PCM reflashed, take it back to the shop and let them know you have this code. Good luck fixing P0605 in your Mazda Miata!