For the price of a USB ALDL cable (around $40-60), you can have dealer-level diagnostics from the early 1990s. Terry may not have a corporation or a tech support line, but his Mode 22 scan tool has kept more vintage GMs on the road than any dealership recall. Download it, connect it, and finally see what your old GM is really thinking.
On your scanner, select “GM Enhanced” or “Manufacturer Specific.” Choose the PCM (engine/transmission module).
The "GM Mode 22 Scan Tool by Terry" (often just called "GM Mode 22 Scan Tool" or "Terry's GM Tool" in online communities) is a that was born in the early hobbyist days of automotive computing. It was designed to work in tandem with the widely popular ELM327 interface and compatible hardware.
Before OBD-II became standard in 1996, GM used a proprietary data protocol called . Within that protocol, "Mode 22" refers to a specific command that requests real-time data parameters from the ECU (Engine Control Unit).
Generic tools show short-term (STFT) and long-term fuel trims (LTFT). Terry's Mode 22 tool allows users to see fuel trim cell data. GM vehicles organize fuel management into distinct "cells" based on RPM and engine load. Pinpointing which specific cell is lean or rich makes tracking down vacuum leaks or failing injectors much faster. 2. Transmission Metrics
