I've had a check engine light on for a while now and the code is P1423. I had it checked and was told to replace the passenger side O2 sensor and to check the vacuum lines. Well I had both replaced and I still have the light come back on. Any ideas what else might be causing this?

 

I would have done the O2 sensor after everything else -- Check the combi valve, make sure the passages are clear, and the lines from the air pump are not cracked.