I got a code po421 on my 99 mits mirage. Brought it to the shop they said it was either the front o2 sensor or the cat converter. I was told if I replace the cat I also got to replace the o2 sensors. If i replace the front o2 sensor and that doesnt drop the check engine light. And then buy the cat converter. Will I have to buy another o2 sensor?

SOURCE: O2 Sensor defect

MR988905 costs $300+

SNG O2 sensor is just below $100

they may claim being made with the same quality and both are tagged "direct fit" for easy installation.

its just my idea, good luck.

SOURCE: I have an '01 Galant

Im pretty sure the bank 2 is talking about by the catalitic converter and since is sensor 1 it should be before the CAT otherwise it should be on the exahust manifold prolly in the back of the engine.

SOURCE: oxygen sensor 2000 galant

The first sensor closest to the exhaust manifold is for the air to fuel ratio. The second one if it has one will be further downstream and checks to see if exhaust is plugged.

Is your car running right especially at idle????

SOURCE: Spinning My Wheels!! ---- P0421 Code Mitsubishi Galant

I have the same vehicle, same code P0421, changed the BOTH O2 sensors and the check engine light was still on, took it to my fathers garage and checked the Catalytic Converter, and it was ok later I found out that P0421 means that the vehicle doesn't have the proper mixture of gas and air (running richer) so I changed the MAX air flow sensor and wala the code went away.

SOURCE: engine boggs down when trying to go to pass geas

Still sounds like a transmission problem. Especially if it is revving up that high on RPM's and going no where. Sounds like slipping or something in transmission. Could be torque converter.