Need MIL Eliminators?

crazyone64 · May 18, 2026

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.
I don't have a scanner advanced enough to read the computer data

Thanks.

squeak93 · May 19, 2026

1: Bosch sensors are more than acceptable.
2. I'm not sure you can buy them anymore. Good chance you can find a schematic still online how to make your own with some resistors and diodes (did it about 20 years ago so no clue exactly what parts to use anymore).
3: If your cats are bad its hindering flow. You can go off road pipe (if you can find one) or replace with a catted pipe and not need a mil eliminator (most likely).

crazyone64 · May 20, 2026

crazyone64 said:
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.
I don't have a scanner advanced enough to read the computer data

Thanks.

Here's my findings with live data:

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

GOvert · May 21, 2026

Have an exhaust shop do a pressure test to see if your catalytic convertors are clogged, replace if necessary, then replace your 23-year-old o2 sensors with new ones. Then the Malfunction Indicator Light (MIL) should stay off after any codes are cleared.

Need MIL Eliminators?

crazyone64

squeak93

crazyone64

GOvert

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