MDTAHOE

For the PO430 it needs a cat converter on the passenger side. Use only an OEM cat. 99% of the time it is a bad cat an O2 sensor will NOT fix it. For the PO308 I would switch #8 plug with a different cylinder and # 8 wire with a different cylinder then the plug clear the codes and see if it follows either. This will eliminate a defect plug or wire. You can not cross plug wire on these truck as there is not a distributor cap and each cylinder has its own coil directly above the plug. If does not follow you need to check for a vac leak on that cylinder (not common for a single cylinder vac leak though). Then look at the fuel injector, you need to remove it from the rail and inspect it. #7 and #8 injectors are common for getting clogged on these trucks. If there is any junk in the rail or injector clean the rail and install a new injector.

For code po430 stop wasting time and money put the cat on it will fix it. Trust me I do performance/ electrical work all day everyday. My personal (not the shops) scan tool cost $14,000. I am one of the best diagnostic Technicians in my area. The shop I work in is one of the best equipped shops out there. Who Is Pat Goss? | Goss' Garage
Our Staff | Goss' Garage PO 420 or PO430 is almost always the cat. I see tons of cars come in on a regular basis with those code that someone (who does not know) has already replace all the O2 sensors on the car and guess what,it was not fixed. The O2 sensors are what is seeing the problem not causing it.