Taking it to Autozone or places like Advanced is a good idea if he has a SES/CEL Mil light on. Our local Autozones will not scan unless the light is on. And the other problem is Autozone uses generic scanners which only give codes such as P0XXX and not GM manufacturer specific codes such as P1XXX. Many ignition, misfire, crankshaft position or camshaft position codes only show up as P1XXX. Although the PCM does a fairly good job of giving DTC error codes it doesn't tell you what exactly is wrong or what to replace. Many times it's still hit or miss and involves troubleshooting or the process of illimination.
For example: P0420 is CAT convertor operating below efficiency. It is generated by the rear 02 sensor after the CAT via the PCM module and referencing to the front 02 sensor. So therefore there's the possibility of a rear 02 sensor, CAT convertor, front 02 sensor, exhaust leak, a defective EGR valve, wiring or PCM module itself.
Many times no DTC error code is generated which makes it even more difficult. Just my 2 cents.
