Have both P0421 and P0431 engine codes at the same time no others show up where to start?

Question

2000 Mitsubishi Diamante 3.5L. Have both P0421 and P0431 engine codes at the same time no others show up where to start?

Answer 1

Yes, using low octane fuel is the #1 cause and #2 is not doing the maintaince and #3 is driving the problem is the car had any.

start by replacing the oxygen sensor first and using 91 octane fuel from Shell

Answer 2

P0421 MITSUBISHI - Warm Up Catalyst Efficiency Below Threshold- Cylinders 1-4
Read more: https://www.autocodes.com/p0421_mitsubishi.html

P0431 MITSUBISHI - Warm Up Catalyst Efficiency Below Threshold- Cylinders 2-3
Read more: https://www.autocodes.com/p0431_mitsubishi.html