i forgot that he is working with a 2300... that will only have a BANK 1 series of sensors.. i rarely see them and i was not thinking.. as the left and the right banks feed into the cat with the sensor 2 output as feedback..

my tests were to look at the upstream sensors to verify that with a fixed throttle at 2500 RPMS the system would be in a feedback loop as expected and when slowly returned to idle the system should maintain the feedback loop.. not flat line the upstream sensor output. if at a fixed throttle 2500.. the upstream sensors are flat lined.. there is a different issues happening.. either the sensors are dead as they are flatlined or there some kind of leak. a cracked exhaust manifold.. uncommanded air from the air injection system.. vacuum leaks... clogged fuel injectors.. low fuel pressure.. its only a test to verify that the rusty upstream oxygen sensors he soaked with spray cleaner that will have destroyed the zirconia surfaces in the oxygen sensor.. if its rusty that rust is conductive and will short out the surface of the oxygen sensor .. so instead of creating signal voltage up and down as the system is commanded rich and lean rich and lean.. to average out the exhaust readings fine tuning the air fuel ratio .. one could also look at the short and long term fuel trims. see if they are operating within 10% plus or minus of Zero. i would hate to see him throw a new cat on it.. when that is NOT the problem. i could tell him how easily get rid of the P0420.. forever but i won't as i am a mechanic who likes to breathe fresh air right out the exhaust . there is a way to get rid of P0420s even if you don't have a cat.. and i will never tell. and it can be done for under $10 bucks. without cutting a single wire or disconnecting ANYTHING. and no i won't share the secret.. because it is a 10,000 dollar fine if i do..