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What the heck is a MID 128 and why is my truck flashing about it?

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Anyone who has spent any amount of time behind the wheel of a truck has seen the dreaded check engine light (CEL). Often we are able to navigate through onboard diagnostics and find some type of fault code. But what do we do with this information? What do they mean?

The first thing we need to know is that these are generic codes and are not really meant to be used for diagnostics. The codes can lend a bit of inside inform as to what is going on inside your truck but you should have it properly diagnosed before you change out a $3000 turbo.

The first number will be the MID. It will be displayed with a three digit number after it (i.e. MID 128). The MID is the code that will identify what ECU or module is reading a problem. For example, if you see a MID 128 you know that the fault code is being sent from the engine (ECM). See the list below for more MID identifiers.

MID 128 – Engine

MID 130 – Transmission

MID 136 – Brakes (ABS)

MID 137 – Trailer Brakes (ABS)

MID 142 – Engine (Some Mack)

MID 158 – Engine (Some Mack)

MID 219 – Eaton Vorad

The second number will be a PID/SID and it will display a one to three digit code behind it (i.e. PID 27). The PID code identifies data or the status. If you have PID 127 on a Volvo it is showing an issue in EGR Valve #1 Position Sensor. If we break down what we have already learned we see an MID 128 PID 27 and we know that our engine ECM is picking up a problem with EGR valve #1.

The final number is the FMI code. This code describes the type of failure the module is detecting. It also will be followed by a one to three digit number (i.e. 003). If we are showing an FMI 003 the module is detecting an open circuit. Let’s bring this back to our previous example and tie it all together.


MID 128 / PID 27 / FMI 3

This would tell us that our engine ECM is detecting an open circuit on the position sensor for EGR Valve #1 on a Volvo D12 engine.


But wait, how did you know it’s a Volvo D12 EGR valve you may ask. First off it’s my scenario and I can make it whatever I want. Second, this is where the problem comes in with generic codes. A truck has multiple vendor modules and all of them need to be speaking the same language to connect and give you the correct output. Unfortunately this is not the case. Each truck is different, but the message from the engine may go through any number of modules including the VECU (vehicle electronic control module) before showing up on your dash. It’s like a really bad game of telephone, you remember the game. Your class lines up and one kid says “Open circuit at your wheel speed sensor” and by the time it gets to the final kid its “Your engine needs an overhaul. Oh, and Jenny thinks your cute”.

The main function of each module is to monitor the area of the truck it is responsible for and let the driver know if there is a problem. When you take the truck to the dealer they are able to hook the truck to the proper diagnostic equipment and communicate directly to the specific module and find out first hand from the source.

The fault codes can be a great tool when you are on the road, but keep in mind the best way to diagnose a problem is to have it checked by a technician with the proper diagnostic equipment.



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