Please help with dtc po 420
I have a 95 Blazer 4.3l with 166,ooo on the clock. My code reader indicates po 420 and the service engine soon light is on. It has no driveability problems. My question is the o2 sensor bad? If so which one is it? My guess is the one before the cat. I replaced the one after the cat at
100,000 miles. Can anyone help? Thank you.