You aren't going to be able to diagnose this until you reinstall the catalytic converter. The Engine Computer uses the oxygen sensor behind the converter to monitor its efficiency. With no converter to clean up the exhaust, the rear oxygen sensor will report that no change is taking place to the gas as it passes through. That will cause a diagnostic fault code to set after a short period of time related to "catalytic converter efficiency". Since there is a known problem, that can't be relied on as a reference for other tests, so those will be suspended. The Engine Computer may not detect a malfunction that would lead you to the circuit or system that needs further diagnosis. Without those fault codes or test results, you could spend weeks trying to figure out where to start looking.
Misfiring is usually due to worn spark plugs and wires, especially when the problem is intermittent. A spark-related misfire will result in raw fuel going into the exhaust where it will burn inside the catalytic converter and cause it to overheat. If the catalyst melts, it will stop working and could be the cause of the original fault code. This is where ignoring a minor problem can turn into an expensive one. That's also what can happen when the "converter efficiency" code is set which turns on the Check Engine light. Since the light is already on, you'll never know if another, totally unrelated code sets that is related to something else that could turn expensive if ignored unintentionally.
Monday, October 26th, 2015 AT 7:33 PM