I have a 2004 Kia Optima with 4 cylinder 2.4 engine at about 64K miles. About six months ago, I replaced a crankshaft position sensor and while I was at it I put in new timing and balance shaft belts. It ran fine until last weekend when it stalled. Scanner code said it was the camshaft position sensor so I replaced it. It stalled again after a couple of days and again gave me the same code. It started again and ran fine all day so I cleared the codes. It’s been running fine but the check engine light came on again. Now the code it gives me is PO421 Warm up catalyst below threshold (bank 1). What is this and how to fix it.
This means that an O2 sensor (downstream of the catalytic converter) is reading that the converter isn’t working quite as well as it should. It may be that the converter isn’t doing the job; alternately, it may be that the downstream O2 sensor isn’t reading correctly. Testing for the correct voltage at the sensor will reveal whether it may be faulty.
Supposedly this code can appear if the car’s had a lot of recent cold starts. If you know someone with a code scanner, have this code cleared and see if it comes back. If the converter and/or the sensor are actually faulty, they should be covered by the federally mandated 8yr/80K emissions warranty.