I backed up in a dirt lot and heard a metal scraping sound. After driving about 30 miles, the check-engine light came on with code P0420 indicating "Catalyst Efficiency Below Threshold (Bank 1)". I inspected the bottom of the car and found a scrape mark on a section of an exhaust pipe. There is soot deposited near a presumably loosened connection of exhaust pipes. The possible reasons for P0420 are:
- Bad O2 sensor
- Intake vacuum leak
- Bad spark plug
- Bad catalytic converter
With the car running well, I don't think it is due to reason 2 or 3--I hope it's not reason 4. Can I measure upstream and downstream O2 sensors using a voltmeter? If I could, what are the normal functioning upstream and downstream O2 sensors' values? Thank you to whoever took time reading/answering my question.