P0420- catalyst effciency low bank 1

Probable cause catalytic converter defective

Engine misfire or running condition (no fowled plugs or oil on plugs, no misfires)

Engine oil leakage into exhaust valve guide seals, piston rings (god, I hope not)

Large vacuum leak ( no vacuum leaks, but muffler has a large leak)
Exhaust band rotted off, and where it sat on muffler is rotted through. Didn't leak till the band came off.
So can a bad exhaust cause this? Also it died once, ran until hot, shut down 5 minute, started back up, and died.

 

You should know if you use oil or not. Have to use more than a quart per 1500 mi to really cause that

If it's running good and no codes, I suggest you recently bought gas at a 'corner convenience store' station. Did it run fairly hinky for a while?

Had one go from working fine, no rattle to a ball plugging the outlet in 100miles after a tank of Citgo from a seedy Mom&Pop.

Whatever, unlikely the O2 but if you want, you could change it first

 

P0420 - Catalyst System Efficiency Below Threshold (Bank 1)

found this using the search feature

 

So everybody knows, a bad leak in exhaust system can cause a p0420. I changed the muffler, and in a matter of a mile the code cleared it self. CHecked with scan tool and came up clear.

 

The very first step of the flow chart in post #3 has you check for exhaust leaks. When the catalyst monitor runs its test, the PCM momentarily commands a full rich fuel mixture and then a full lean mixture immediately following. The PCM then watches for a change in the downstream oxygen sensor voltage within a specified time frame. If the O2 sensor reacts too quickly to the changes in fuel mixture, P0420 is set. An exhaust leak in front of the converter can, (and usually does) cause the P0420 to set, due to the artificial lean mixture being supplied to the converter. If there's an exhaust leak after the sensor, it will not set the DTC. The sole purpose of the downstream oxygen sensor is to monitor the catalyst, that's it, that's all it does.