I am the proud owner of a 1998 Celica GT hatchback with an automatic transmission and the 2.2L engine, boasting 217,000 miles. I’ve owned the car for 18 years here in Texas, and it’s all original, with no modifications. Aside from routine maintenance, it has been remarkably trouble-free.

A few years ago, I encountered a P0420 code and successfully used your lacquer thinner method to resolve the issue. Recently, however, I’ve experienced overheating, which triggered a new P0420 code. I want to mention that the radiator, hoses, thermostat, and coolant were all replaced with new OEM parts in January. Despite these updates, the engine is now running rough, and I want to make the necessary repairs.

I’ve noticed that my Celica has a slightly different setup compared to the 1994 model, featuring a Toyota Three-Way Catalytic Converter (TWC). It has a front converter with both upstream and downstream O2 sensors, and a rear converter without any sensors. I was surprised to find the front converter, given that this is a federally compliant vehicle and not a California model.

Considering the mileage, I’m unsure how best to proceed. Should I start by replacing the Denso OEM sensors? Should I replace the front converter, the rear converter, or both? Or would it be best to replace everything at this stage?

I appreciate any guidance you can provide. Thank you for all the valuable tips, tricks, and creative content you’ve shared over the years!