Hi,
2003 mach 1. 58000miles.
Service Engine Soon = P0420 = Catalyst System Efficiency Below Threshold Bank 1

1. Any recommendations for the replacement brand of o2 Sensor amongst Motorcraft, NPK or Bosch?
2. Any recommendations where to buy already made, replacement MIL eliminators?
3. Any way to confirm if I need MIL eliminators other than waiting for a code to popup?

Exhaust system consists of Ford shorty headers, MAC H-Pipe, Flowmaster axle-back. I think cats are stock, but not 100% sure.

Here's my findings:

Sensor 1 (upstream)
Bank 1 (passenger) = 0.135 - 0.880
Bank 2 (driver) = 0.170 - 0.850

Sensor 2 (downstream)
Bank 1 (passenger) = 0.015 - 0.635 (P0420 Code)
Bank 2 (driver) = 0.015 - 0.640

Thanks.