Ignition coil, P0421, catalytic, should I replace 02 too?

[blue_and_shiny]

Long-time listener.

I've a 2016 CX-5 AWD touring with 158k miles. About 15 months ago, my 2nd ignition coil failed and the original (cheap) mechanic put a non-oem part in. It "ran," but a few months ago, the check engine light started going on and off with P0421. My (expensive & reliable & far away from me) garage has replaced the ignition coil with an oem part, but the code still returns (somewhat intermittently, and the engine runs okay though gas mileage is reduced).

They've recommended the catalytic converter be replaced and they've quoted me a price, but I can't get them nailed down on ordering and doing the work. And, they're far away. I'd kind of like to explore a local (5 star) garage, and they said they'd replace the part for much cheaper, if I get the part myself.

My question is, should I/do I need to also replace the upstream O2 sensor? The original garage who did the diagnostic (they checked for air leaks in the system and inspected it pretty thoroughly) said it was pretty clear the bad ignition coil had damaged the catalytic, but to really get it "fixed," do I have to also replace the O2 sensor?

 

[Lazy2.5]

How much oil does your engine consume (if any.)

what's your fuel economy like?

can you post a pic of your exhaust tips?

are your spark plugs in fair condition?

 

[Lazy2.5]

I would suggest to go ahead and replace the exhaust manifold sooner rather than later. Save your time and just do it with a local mechanic. Order the manifold yourself.

You will also want to get your second cat checked. Either delete it all together or replace, as i'm sure it will be in a similar state.

 

[blue_and_shiny]

@Lazy2.5: the engine runs great. It doesn't burn oil. I've had a decarb on the engine done every 25k miles since I bought it new. Fuel economy was 450+ miles to a tank but now is running just under 400 miles per tank. Spark plugs were replaced about 15K miles ago. I'd have to go out and grab a photo of the exhaust.

In general, the car is in great shape and I want to keep it that way. I'm definitely going to do the exhaust manifold as soon as I buy the parts, but I'm wondering if it makes sense to put in the $200 O2 sensor at the same time? Or is that overkill?

 

[Lazy2.5]

@Lazy2.5: the engine runs great. It doesn't burn oil. I've had a decarb on the engine done every 25k miles since I bought it new. Fuel economy was 450+ miles to a tank but now is running just under 400 miles per tank. Spark plugs were replaced about 15K miles ago. I'd have to go out and grab a photo of the exhaust.

In general, the car is in great shape and I want to keep it that way. I'm definitely going to do the exhaust manifold as soon as I buy the parts, but I'm wondering if it makes sense to put in the $200 O2 sensor at the same time? Or is that overkill?

That's unusual then. The pic of both exhaust tips should provide some insight on your air:fuel ratios to estimate if that has something to do with it.

gasoline quality would be the only other factor I could think of. you could try to run some good fuel system cleaner for 1-2 tanks to hopefully ensure this is prevented in the future.

Do you do alot of city driving?

I would definetely do the o2 sensor, yes. it cant' hurt .