P0420 (and P0430) codes are NOT due to O2 sensors. The sensors are run through a self-test BEFORE the Catalyst Efficiency test is run. Don't waste your money replacing sensors that aren't bad.

As mentioned, these codes come up when the cats are no longer doing their job at the required minimum efficiency level.

Options are:

Ignore the CEL

Install MIL-eliminators to fool the PCM into thinking the cats are working

Replace the cat.

Your selected course of action will depend on whether you have emissions testing, your budget, and you willingness to comply with federal air pollution rules/laws.

You can test the cats in-vehicle with a scanner that allows you to graph the output of the upstream vs the downstream sensor in real time. A dead cat is easy to spot as both sensors will switch almost simultaneously. A good cat will show the upstream sensor switching regularly while the downstream sensor is slow to switch. The cat monitor program compares the ratio of upstream to downstream switches and, when the ration falls below a preset minimum, sets the P0420/430 code (s).

Steve

 

I have not had truck to muffler shop yet ....have talked to them over phone ...they are saying i should have two cats on this truck @ 1996 with 4.9 and that the one thats bad is probably the one between the sensors ,,,does this sound right??? Thanks

 

A cat has to be "between the sensors" in order to be monitored.

Ford has quite a few models that have an additional cat downstream to further improve emissions results.

Since I've never looked at a truck with your configuration, I'll leave that to you and your shop.

Steve

 

V8 engines have 2 cat 'assemblies', one for each engine bank.
These go to a 'Y' pipe then into the muffler.
The muffler plays no part in the monitoring by the sensors.
This means you have a total of 4 sensors.
One ahead of each cat assembly and one in the "middle" of the last cat or behind each assembly.
These asemblies are made as one, and must be changed that way.
If one side is bad then change the whole assembly.
FYI, there are Federal rules the shop has to observe in changing this assembly under penility of law. Alot of money in fines.
Additionally you cannot even go to a bone yard and buy a cat assembly under the law. They are is not supposed to resell them.
The reason is they cannot be tested before sale and then there is no certification they are good and the right ones for the application when it come to emmisions testing, cost etc.
There is no legal way to get around it unless you actually make the change, then 'you' accept the results of that action.