Reversing GM OS 12588500

This project is a work in progress and will be updated regularly as my research continues. 

Operating System Reverse Engineering - GM OS 12588500

This project started with a simple goal: I wanted to integrate a wideband oxygen sensor into my 2004 Monte Carlo SS so I could more accurately log AFR for tuning. The factory PCM only expects a narrowband O2 signal, which reads between 0 and 1 volts and just indicates whether conditions are rich or lean. A wideband O2 outputs a linear 0 to 5 volt signal that shows actual AFR across a broad range. That difference meant I couldn’t just wire it in directly.

Once I realized this limitation, I started looking into whether it would be possible to make the PCM read and process a wideband signal. That led me to research how the PCM handles AFR internally and whether I could rewrite the functions involved. I also documented the full wiring harness to see where I could insert a signal and identified extra unused inputs that could potentially be repurposed.

From there, the project grew into a full reverse engineering effort. I mapped all available pins, looked at how the PCM processes inputs, and began working in Ghidra to map every table and dataset inside the tune file. I labeled key calibration data like the MAF table, fuel trims, and AFR targets, and used those as anchors to start understanding the functions around them.

At this point I’ve mapped all of the tables and values in the binary and I’m using that map to figure out what the different functions are doing. This work is building a foundation for future custom firmware mods that go far beyond what normal tuning tools allow.