Feedback on FCAR F502 heavy duty trucks

Tested FCAR F502 code reader on a few heavy duty trucks:

OK experience:

2000 kenworth t 600: scan codes with a 6-9pin connection

sterling Mercedes Benz Diesel: working fine

Allison Transmisssion on a 2001 navistar-international(School Bus): perfect, clear codes and read codes

K.O. experience:

ISC 960 model or GMC Savanna school bus: cannot hook up.

In sum, it’s nice when it works. This is a code reader only and not a programmer it will clear codes and read codes, nothing else. Works great for the purpose it’s built for. But it would be better if the factory improves the hardware design coz the display is very small and dim. Extremely hard to read.