P0420 and P0172 Codes for 1997 Chevy Astro with 4.3L engine

Question:

This van has codes P0420-low catalytic efficiency and P0172-rich mixture bank 2. The fuel pressure is within limits and has no vacuum leak. The pcv valve was taken out and the reading to the downstream oxygen sensor did not change. Removing the dipstick tube did not reveal any blowby. What can I do before I bring this to garage?

ATS Adviser:

On this engine, you have 3 oxygen sensors. Two of them are in the exhaust manifold(upstream) and the last one is at after the catalytic converter or CAT(downstream). Try monitoring the 2 upstream sensors first by introducing vacuum leak and check the signal on the sensors. If the reading are not going down when you do this, replace the 2 upstream sensors. Reset the code after replacement and see if the down stream reading will now improve.

Note:

The 2 upstream sensors are used to monitor the engine whereas the last one is used to monitor the CAT. Contact ATS to get more help if the code remains.