i'll type this twice because of how important it is (for any vehicle).
check your grounds.
check your grounds.
it is rarely the ECM/ECU (computer)
start with the main negative cable from your battery and work your way around the engine bay. remove, clean with sand paper and reinstall all of them.
i've even had one of our vehicles give a EEPROM error code (mechanics said it needs a new computer). it was running like dog **** and i limped it all the way home. it was misfiring so badly that the exhaust manifold was almost glowing because of the fuel getting dumped and ignited in the exhaust.
i decided to go back to the fundamentals stuff and upon inspecting, i found a loose ground wire that was coming from a harness to the intake manifold. the bolt was finger tight so i removed it and cleaned it with a rag. reinstalled it and the car fired up and ran perfectly for another three years until we sold it.
what are the symptoms?