2007 Pontiac G6 GT question...I have been pulling a P0420 code for a couple months and no matter what I try it will not go away. I have changed the plugs, the fron 02 sensor and the passenger side catalytic converter and I wanted to try and find an answer before I spend anymore money.
The car drives fine, even better now with the new cat, but now I am pulling a P0174 code along with the P0420. If anyone can shed some light on my situation it would be much appreciated!!
How many engine miles & which engine?
Did the 174 pop soon after you did some engine work?
At first glance, the issues appear unrelated, as they are on diff banks.
So far you've replaced the bank 1 cat & the bank 1 pre-cat oxygen sensor.
For the new P0174 (bank 2 over lean threshold):
First clean the MAF sensor if you have not.
If no joy, it's likely you have an intake air leak post MAF in bank 2. A smoke test is often successful in finding intake leaks. Possibly a breach in the manifold gasket or head gasket.
If you have done any engine work or mods, revisit those closely.
A scan tool with live data would be very helpful in analyzing both of these issues.