Contour Enthusiasts Group Archives Forums General Troubleshooting Upstream and Downstream O2 Sensors

I understand the upstream O2s control fuel /air and should be replaced if mileage goes down

What do the downstreams O2s do? Only light up the CEL ? Do they CONTROL ANYTHING ?

I want to bypas them as they keep the CEL light on every 3-4 days

The downstream sensors are there to monitor catalytic converter efficiency. they do this by switching at a slower rate than the upstream sensors provided the converters are good. once the converter fails to do it's job the downstream will switch at a rate similar to the upstream sensor which triggers the orange light & codes p0420 or p04X0. This system was part of government mandated obd II requirements. They are known as catalyst efficiency monitor sensors.

THANKS

I have been reading that for $20 I can replace the downstream O2s with Mil Eliminators

Is this ok ? As they perform no "CONTROL "function at all

Provided local laws dont require emission checks they will probably fool the computer. Are you sure that converters are free & not plugging up? Engine has good top end rpm's?

Phone your ford dealer with your serial vin # and ask them what the coverage is on your model.

I read a lot online and these downstream O2s might calibrate the upstream O2s

I bought a new Downstream O2 and will see this week what hapens. Next is MIL eliminators and GUT the CATs

At the same time i would remove the mass airflow sensor from it's housing & carefully clean the hot wires with brake & choke cleaner.

You didn't say whether or not you checked for catalytic converter coverage. I don't know the exact years but ford has a unadvertised program to cover converter replacement on these cars.

the downstream o2s only monitor the efficiency of the catalyst on the bank they are mounted to. they don't "calibrate" or do any other adjustments to either the front o2s or the fuel mixture. Gut the convertors and install simulators.

THANKS

So far the light has been off for 6 days with new O2









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