New Codes?? P0420

just got my van fixed for other problems now I'm getting a p0420 code....i was thinking it may be O2 sensors I replaced the ypipe and cat not that long ago but just re-used the o2 sensors maybe it's time for new ones or could something else be causing this......just did a trip with over 600miles and was averaging about 21MPG. Will see how that does with my travel trailer this weekend.

Another thing i noticed that was weird was that when running my vent or AC as I climbed hills it would cut off then as I crested the top of the hill the air flow would return? any thoughts?

DTC P0420 relies heavily on all of the O2 sensors for checking the converter. If the sensors are slow in reacting to changes in fuel mixture, the P0420 can be set. Here's a quick test: Connect the scan tool, bring engine to operating temperature, maintain 1,500 RPM and watch the "bank 1 sensor 2" voltage. It should be remain between .2 and .4 volts. Post your results.

The HVAC problem is caused by a vacuum leak. Could be a hose, the check valve, the reservoir, the switch, or an actuator. The vacuum line from the PCV valve to the HVAC check valve is a common problem.

ok so I think I've got the P0420 code figured out.....there was a nut missing from the exhaust manifold that the cat y pipe bolts onto causing a slight exhaust leak before the cat....I replaced the nut and added some muffler cement to the joint and cleared the code and bingo....no return of the code.....

Thanks for the info about the HVAC I'II check it out!

Any exhaust leaks before the converter can also set the P0420... good job!

Completing a drive cycle will force the monitor to run. Just driving may take a while for it to run.
http://www.obdii.com/drivecycle.html

Post your findings on the HVAC issue.