Having both P0430 and P0420 codes. I verified the oxygen sensors, air leaks etc. All turned Ok. The mode 6 in forscan was flagging oxygen storage capacity was out of specs on both cat converters. So I changed both cat converters. That fixed the problem and I could pass the emissions and renewed the car registration.
Lessons learned:
*) Radiator side cat is fairly direct, without anything special. What I would highlight a 1/4 small torque wrench, to fit easier in the tight places
**) Rear /firewall cat is more complicated with AWD - like my car. FWD would be easy as there is no PTU to make life more complicated. Each manifold to head bolt will require its own set-up of long extensions + swivel to reach. What will be a challenge is to drop the cat out.
There are many opinions on AWD models to drop the rear cat. I saw one person that removed the Hub from the PTU (towards the driveshaft) to remove the cat. Others claim the ford manual says to remove the passenger CV axle. I found one youtuber that pulled the engine slightly forward with a ratchet rope - enough to get extra clearance to get the cat out in between cv axle and the frame. The driveshaft did not need to be disconnected, the bolt holding the bottom transmission mount to the frame need to removed, so the engine can be pushed forward.
1/4 torque wrench is preferable. I had a 3/8 torque wrench and was big enough to struggle finding a good position to torque the nuts. I did not want to transfer torque from the wrench via universal joint, so I was looking for a straight line, at the expense of having "one click" play. Painful.
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As a note. Wanted to avoid initially the OE dealer units. On the front/radiator side, I tried initially an Amazon purchased Walker cat. The welding s were so off, that the Y-pipe did not fit at all. The bolting was impossible. Returned it to Amazon, and got a Dorman unit from Autozone next day (as it was Sunday). The Dorman fit slightly better, but the fitting position put the Y-pipe in such a crazy twist, that i decided to return it as well and go with the dealer cat. Dealer part fit perfect.
On the rear cat, I gave a Dorman from Amazon a try, knowing i can return it easy if it does not fit again; so I took the risk, given half price. This one fit quite OK. So I kept it. Mode 6 readings show the Dorman (rear) oxygen storage capacity is way worse than the dealer OE front cat. Transition from lean to rich and rich-> lean are way worse in the Dorman. True, within specs, but still. But it helped me pass emissions..... I do not know for how long it is going to survive
