'03 3.8 P0420 code


New member
OK, so I'm experiencing the dreaded P0420 code and I'm trying to diagnose without randomly buying parts, I have an OBDII reader and I can read and reset the code which will come back after about 50 miles of driving.

I have checked the catalytic converter inlet and outlet temps and I get a nice 120 deg temp rise which to me says the problem isn't the CAT, if it's heating up from low 300's to well over 400 deg.

The O2 sensors seem to be functioning as well. The up stream sensor reads low to high like it should from what I've seen. It oscillates between 0.05 to .9 volts. The downstream sensor will oscilate initially but then stabilize at a very steady .74 volts. Is that a bit high?

I searched/inspected the intake manifold and vacuum lines for any signs of a leak but I haven't found anything.

Any advice or suggestions would be helpful at this point.


Staff member
You seem to have the situation well diagnosed, your cat difference reading of 120 is good. You dont mention the mileage but if you are past 100K and the O2 sensors are original I would try changing them using an AC-Delco part.


New member
170k miles...the downstream sensor looks like it's original. but the numbers look pretty good. The upstream one is protected and looks good physically but it could be original. bought the car used a couple years ago.


New member
Changed out both sensors, upstream and downstream. also sprayed around the intake and vacuum lines with carb cleaner with engine running but couldn't find a vacuum leak. I reset the codes again and did the drive test....drats! I'm still throwing the P0420 code. What am I missing?