Originally Posted by toyomoho

Speculate the only a way a converter could cause an issue is if was plugged up. Here a vacuum gauge can check for excessive exhaust back pressure. Read up on checking exhaust system back pressure.

Any other codes showing up such as P0171 or P0172? How does the engine run and idle? Has the MPG gone down? Any gas smell in exhaust?

Below is the list of possible issues.

Open/short in wiring or connector issue.
Vacuum hose leaks.
Ignition. Do a spark test and compare to another cylinder If you can swap coils with cylinders try this and determine if the code moves.
Injector.
Fuel pressure. This needs to be done by attaching a pressure gauge.
MAF. Try cleaning the MAF
Engine Coolant Temperature sensor (ECT). Temp between 167 and 212F
Compression.
Valve clearance or timing
PCV valve, hose and connections. Check this first.
Intake air leaks
Also check for exhaust leaks. Check the flex pipe on the exhaust for leaks..
ECM

A decent scanner can check real time coolant temp and MAF readings. Some can read how each cylinder is operating..