Hello everyone. I have a 2004 Chevy Blazer and have been having issues with a P0420 code. I checked the sensors and they were bad and replaced them; however, the code came back. any suggestions? the Cat looks good, there are no leaks that I am aware of...

 

Did you ever experience:

misfires
leaking injectors

Has something been changed before the code was set such as:

distributor
spark wires
spark plugs

And how do you know that the CAT is good from looking at it?
The diagnosis would involve looking at the signal from the front and aft O2 sensors directly. That is usually what the ECU does to determine if the CAT is good and if not will set the fault code P0420.

Which sensors did you change? (I guess the O2 sensors ahead and aft of the CAT - but just to make sure we get the whole picture)

 

if you have an evap mil or misfire, also check seal ring on intake at throttle plate, any air leak after maf can over lean system and doesn't always toss a code but will hit you up for 02 bank 1 sensor 1 or 2 and 2. Leaking intake will really lean the system and mess w the fuel trim tossing 02 light blown fuse for 02 sensors will do the same. Using a laser temp gun super cheap at just about anywhere to determine cat function one is run to closed loop, laser top of cat temp should be in the 400 range middle 600 to 750 output should be slightly cooler, also w a scanner actron or foxwell or better. Veiw live data to see what the monitors are doing in real time. Gms are known for o2 issues in wiring connectors and grounding. When viewing live data if the sensor reads zero or high 900s its a wiring issue slow response is probably a broken single wire if sensor is new or previous lean conditions may still exist.