Please note, I'm a shade tree mechanic, just trying to help out. Friend has been battling this issue for too long. CEL came on last year. He took car to a shop, they pulled codes & replaced one or more O2 sensors (he doesn't know what the codes were, but said they did "replace" the A/F sensor). Light came back shortly after work was done, but as they gouged him for what I believe to be poor service, and he has limited income, he has continued to drive the car. He came to me, and I got a p0031 (Heated Oxygen Sensor Heater Control Circuit Low B1 S1). The "new" A/F sensor in his car looked terrible; heavy build-up on sensor itself (although this might be normal after 1000's of miles), a poor electrical tape job all the way down the wires, and the wiring appeared to be a bit short anyway, as there was not much slack. Did a lot of research on this forum & others, & hopefully purchased the correct sensor (matched to VIN), Denso 234-9041. Replaced sensor, & CEL came back on after a few days, when car had gone through enough drive cycles. Now getting p0420 (Catalyst System Efficiency Below Threshold B1). Again, low on cash, here's the stupid question...better for him to get the emissions checked pre- & post-cat, or replace the rear O2 sensor? Be gentle everyone : )

 

The emission converter may be warrantied by EPA regs much longer then the factory warranty. Check the emissions warranty info in the owners manual and determine if the converter is still under warranty.