Gm Mode 22 Scan Tool By Terry -

While it will read standard "Check Engine" codes (Mode 9), it also accesses manufacturer-specific codes that generic tools miss. This includes transmission codes (on vehicles with electronic automatics like the 4L60/4L80E) and body control module codes.

Example 1 – Rough idle on Duramax

Example 2 – Poor boost / turbo lag

Example 3 – Transmission slipping

View every analog and digital input the ECM sees:

To use Terry’s Mode 22 tool, you need a few simple components:

Since Terry’s original software was distributed via BBS (bulletin board systems) and early websites in the 1990s, it lives on through the GM tuning community. You can often find the executable and required cable schematics on: gm mode 22 scan tool by terry

Many modern users have migrated to TunerPro RT because it supports the same Mode 22 data stream but adds a modern interface, better graphing, and the ability to tune the PROM (EPROM) chips. However, purists still keep a copy of Terry’s original lightweight DOS tool on a bootable USB drive.

This is where Terry’s tool beats any generic scanner. You can command:

If you’ve spent any time under the hood of a late-90s to mid-2000s General Motors vehicle, you’ve likely run into the limitations of generic OBD-II scanners. They’ll give you a check engine light code, but they won’t tell you why a 4L60E transmission is slipping or why the ABS light triggers on a bumpy road. While it will read standard "Check Engine" codes

Enter GM Mode 22. According to veteran technician Terry, “Mode 22 isn't just another data stream—it's the backdoor into GM’s proprietary brain. If you don't know how to use it, you're diagnosing blind.”

Here’s everything you need to know about using GM Mode 22 effectively.