1997 Lincoln Towncar "check engine light - code P430"

It actually has 4, O2 sensors. One just below where the exhaust manifolds bolt to the exhaust system then one just in front of the convertor and one right after it. The reading for a PO420 and PO430 is low efficiency at bank 1 and 2
which could also be a bad convertor. However, the sensors are cheaper.

it is the first oxegen sensor after the cat converter yes easy to change unplug wire unscrew from threads and screw new one in

On the money side of things I have bad news for you
The car is far from new,at 15 years old

That is about the life of exhaust converters

You have 2 Codes P0420 & PO430,those are
converters below efficiency

Also more troubling is the ignition misfires

You likely have had them possibly for years
and unaware of it.
Misfires cause raw fuel to enter the converters and
overheat and ruin them. Your are due for replacement
anyway

BUT--you have to work on correcting the misfires first
Then you will need 2 new converters
All 4 O2 Sensors ,as they will be ruined from the heat
or just over 100,000 miles.

You are 100% correct on all your statements
Spark Plugs very seldom misfire and do not correct
anything
You have to diagnose those issues with finding
the cause of the misfire. As you said a combination
of things. Worn plugs,old O2 Sensors,clogged converters,
EGR Valve issues,vacuum leaks. All minor until you
combine them

Let us know how your proceeding or what you want to do

last year doesnt count. What was the code? the oxygen sensor is cheaper than the converter. Have a dealer check it. toyotas are best with the converter from the dealer. so have them confirm the problem.

It won't mess up the a/f ratio. The Check engine light will be on because of the rear O2 sensors. To keep the light off you will need to install O2 sensor simulators.

these O2 sensors are cat convertor check sensors. id be checking cat convertors and hopefully you should try replacing if not all ready is the front bank 2, O2 sensor
if you have scanner can you see fuel trims in data display to confirm high fuel trim only on bank 2 either +-5 or more indicates problem and if bank 2 sensor 2 has fixed high 700mv reading cat no good(warm idling)

I would try to clean out the egr valve , check for leaks at the evap canister, check the evap canister soleniod , and lastly change out the cat convertor, The cat convertor is most likely under the federal emmission warranty of 8 yrs,.80000 miles. You can take the vehicle in to have checked by local dealer or call gm customer service to check the warranty on the cat. convertor by giving them your vehicle number.

You have a V6 engine, bank one is the row of cylinders on the passenger (right) side, bank two is the row of cylinders on the drivers side (left). Your codes indicate an inefficiency in the cat on the left side but this could be due to a bad O2 sensor or an engine that is out of tune and overloading the cat. This car seems too new to have a bad cat. I would change the plugs and the O2 sensor after the cat before going any further. The reason I say the O2 after the cat, it's the one that is critical in sensing cat efficiency. What is the mileage?

its the O2 sensor for sure. normally, its good practice to change the sensor when changing out the muffler and cat system. doing it all together will prevent the sensor from faulting.

I think you will find the aftermarket Cat Conv. is the source of your problem, but without hooking up to the factory test equipment at my disposal at my Ford dealer this at best and educated guess based on 20 years of emission related work, I am a Ford certified master electronics technician