PO420 and PO135 Fault Codes: Diagnosis and Repair for 2000 Chevy Prizm

My 2000 Chevrolet Prizm has had the check engine light on for approximately two weeks. I visited several mechanics who initially suggested replacing or cleaning the mass air flow (MAF) sensor, citing a fault code they believed was related to it. However, I did not receive confirmation of the exact code at the time. After researching online, I discovered that the part itself can be purchased for as low as $143 from mytoyotaguys.com — significantly less than the average $180 charged by some dealerships, which themselves source parts at an average cost of about $77. While markups are common in retail, this discrepancy raised concerns. I decided against purchasing and installing the part myself to avoid potential installation errors. Instead, I consulted a mechanic who correctly identified that the issue was not with the MAF sensor but rather with the oxygen (O2) sensor — specifically code PO135. After replacing the O2 sensor, the car's performance improved noticeably, with better power and responsiveness. The mechanic reset the vehicle’s computer system for $300. However, the check engine light reappeared the next day. I then took the vehicle to AutoZone for a free diagnostic scan, which revealed two fault codes: PO420 and PO13-5. I am now seeking advice on how to proceed with resolving these issues effectively and affordably. The car is due for inspection by the end of the week, and while the light was temporarily turned off via AutoZone’s reset, this is only a short-term solution. Any insights into what PO420 and PO135 mean in a 2000 Chevy Prizm, or recommendations on next steps (such as further sensor replacement or system diagnostics), would be greatly appreciated. Thank you for your help. Paul