Hello and welcome to Just Answer!
The O2 sensor will not cause the engine to high idle like that! The O2 sensors can however cause the P0420 code to come up.
Fault code P0420 can be caused by a faulty catalytic converter, exhaust leak, engine burning oil or faulty O2 sensors. These engines were bad for breaking exhaust manifold bolts and leaking intake pan gaskets. The first thing I would check is to see if there is an exhaust leak at both manifolds. Check for broken studs or bolts.
Then check for the intake leak by pulling out the pcv valve out of the valve cover and leave it attached to the hose and plug the valve cover hole that the pcv came out of. Then pull the breather hose off the air cleaner that goes to the left valve cover and start the engine. Then hold your thumb over the hose that came off the air cleaner for 5 seconds and then release. You should feel crankcase pressure and not vacuum. When you release your thumb you should be able to see the blow by come out. Then let me know what you find. Thanks, Richard
I have never seen an exhaust leak before the cat cause a periodic high idle. I have seen exhaust leaks cause rough idle if it is before the upstream O2 sensor because it causes it to run rich and kinda choke.
Does your truck have the same power it used to it? I have seen quite a few catalytic converters get loose inside and turn side ways which will cause a lack of power and a rush of exhaust sound, plus it will set the P0420 code. Let me know when you get a chance to check these things. The cat will usuall rattle for awhile before it is loose enough to turn sideways. Thanks, Richard
Exhaust leaks can sometimes be intermittent depending on the engine temperature at the time of testing. Exhaust leaks at the manifold will sometimes seal up when hot.
When you get under the truck to change the O2 sensors bang on the catalytic converter to see if it rattles inside. Let me know how you make out! I hope the O2s solve it. Thankyou, Richard
It is always recommended to change both O2 sensors since this code can set by using one new sensor and one old sensor. Both O2 sensors have to work properly or this code could still set. The signals from both sensors are used for testing the converter. When these sensors do not come out I cut them off and use my power hack saw and cut through the threads in two places oposite of each other and dig the thread out. Sometimes I have good success that way if you can get at them. Sometimes you have to break the sensor off and use a 6 point socket. When you get it out and if the threads are damaged you will have to use a O2 sensor tap to run through threads to clean them up. Good luck with replacing them! Let me know how this goes!
I can't remember if I ever had to drop the exhaust on a Dakota to remove seized O2 sensors before. Sometimes using an acetylene torch to heat up the piece that the O2 screws into will help remove them but it can also cause the pipe to twist in that area. If you can't remove them you will have to drop the exhaust to be able to remove them. With the exhaust removed you can use an impact on the O2s to see if that will help. I mentioned before that sometimes I cut the threads out. If you could grind the O2 off without nicking the gasket surface and knock the center out you could use a reciprocating saw and a small metal blade to cut through the threads in 2 places and pop the threads out. I am sorry I can't be more of a help with this but when they are seized it can be very difficult to remove.
When you cut the wires the O2 sensor signal to the PCM goes to .5 volt on a 0-1 scale.
The air fuel ratio will not be fine tuned because it is already at .5 volt and that is what the PCMs goal voltage is so it doesn't do anything about it to try and correct it. Let me know how you make out! Good luck! Thanks, Richard