My 2001 Civic HX was bought with the previous owner saying it “just needs a clutch”. I’ve owned a few 7th gens now so I figured that was an easy fix and despite having 200k miles; it had a permanent tow bar attached to the front and he told me it was pulled behind an RV twice a year on cross country trips and the mileage is much higher than how much it’s actually been driven. Anyway, the only cold I had initially was an o2 sensor code, which I replaced and it immediately went away but the problem stayed so I figured it would be the converter(that was before I found out it had both 7th gen style converters due to being an HX). So I replaced the top one that’s attached to the manifold and it was broken into a ton of pieces when I got it off so I figured I was in the clear; nope problem stayed. Obviously it could be the second cat underneath too but I’m not sure I don’t want to cut it out if I don’t have to. The car starts right up and idles perfectly fine up until it gets fully warmed up; then you can hear a slight miss and stumbling while idling; and if you try to give it any gas at all it’ll stall out unless you keep revving it and then go back to idling. If you try and drive it at all you have to give WAY more gas than you should and it’ll take off in first decent enough but then it starts bucking really bad. I’m just at a loss because ever since I cleared that one code, I have never been able to get it to show another code no matter what and it’s been 3 years. I used to have a daily so it was fine but now I absolutely NEED the car back for me to work and I don’t know what else to do and don’t want to just throw parts at the car. PLEASE help me out. It more information is needed I can provide that. Also, the ECU has been replaced already as well because I originally thought it might have been in limp mode. Later on today I’ll try and attach a video(if that’s even possible of what it’s doing. This is my 4th or 5th forum post and nobody has even acknowledged them.

 

sounds like it could be a bad engine coolant temp sensor
(the one that talks to the ecu).
it's cheap so just replace and see what happens.
also test your tps calibration.

 

So, I just pulled out my top o2 to test the cat(I wanted to pull the 2nd one at the bottom of the bend after the manifold but couldn’t get to it and my 3rd o2 wouldn’t have done anything to check the cat since that’s actually inside the 2nd converter) and the cars issue actually seemed fine. It was loud but it was at least driveable lmao so I’m thinking it’s 100% the converter now but the whole issue is finding the right one because it seems like even though my HX has both style converters(the DX/LX manifold style and the EX underneath style) but the second one isn’t the same as the EX one. I guess that’s because the EX has a different manifold but it’s really annoying. It seems like the angle to go to the bottom of the manifold piece, and the other end on mine has the flange directly after the converter whereas the EX looks like it has some space between them. It’s just really stupid to me that they’d make the HX have 2 cats and have the second one ONLY come on that engine(D17A6 I’m 99% sure). It’s just been such a headache and not having any codes to go on at all is driving me crazy but I know my ECU is working fine because I’ve went through unplugging my TPS and the intake sensor and got codes perfectly fine; and I got a code when I took my top o2 out as well and then it went back away when I reinstalled it after testing the car. I let it get fully up to temp and drove it, and the issue never happened at all but I guess it could also maybe be that top o2 but I feel like that would have a code since it’s a A/F sensor. I suck at diagnosing when I don’t have anything to go off of and it’s ridiculously hard finding ANY information on the HX specifically

 

The second cat could be clogged from the first cats broken pieces. Run your vin into hondapartsnow.com and you can find what you need. Then check rockauto or local exhaust shops for a price on a new cat

 

Yeah, that’s something I’ve considered for sure and it’s most likely the other cat. I pulled out my top o2 to see if it got better and the problem basically went away 100%. I should probably pull the 2nd o2 and check just to see since that one’s closer but idk. Do you think that means it’s definitely the cat or could it maybe be that top o2? My code said low voltage I’m pretty sure but that could be since it wouldn’t have gotten warmed up outside the cat. I just know that the top o2 is a wideband for the air/fuel and was thinking maybe that could be what’s messing it up since it happens when the car warms up fully

 

Can you undo the exhaust before the second cat to eliminate the clogged cat possibility?

Oxygen/air fuel sensors as far as my experience don't really cause serious running issues on older hondas. Maybe some bad gas mileage etc.

Car I just did was a 97 and top of cat looked fine... Bottom of cat was almost melted shut. Had super low idle, no power and terrible gas mileage. Had 02 codes and a po420. New cat and old sensors. Runs like a top again.

And if you used anything but a ntk? O2 sensor it could be a problem. Hondas/Toyota are super picky usually

 

it would also be a big difference between a 6th gen civic like that with obd1 and mine with obd2. But I also say that because my top o2(the one I removed and the one that goes directly into the first cat built into the manifold is apparently not a “regular” o2. It’s a wideband one for the A/F ratio) which is mainly the reason why I think it could be that instead of the cat. I can TRY to undo the exhaust before that second one but there’s not much space. You don’t think taking out that too o2 directly after the manifold would prove it’s the cat? Or should I undo the exhaust/take out the o2 that’s closest to the 2nd one but still before it? I have 3 o2’s total. The top wideband one I took out and fixed the issue, then there’s one under that right where the section of pipe under the manifold/cat connects with the spring bolts to the next section which includes the 2nd cat and it’s still part of the same manifold/cat section but directly before the flange. Then the 3rd sensor goes directly into the 2nd converter. You think if I just undid the spring bolts and unhooked it there or took out that 2nd o2 that’s closer would be a better test? I’m just trying to make sure because I recently had to move the car to my backyard which is on an incline and I really don’t like having to jack it up and get under it AT ALL with the car facing downhill lol I’m honestly just going to run to crazy rays(called LKQ now but it’s a string of junkyards near me in MD) and just grab one or two o2’s and test it out just to be safe. Only issue is that they just cut all the manifolds off to take the cats but I’m hoping since the EX cats are underneath, they wouldn’t have touched that top o2 unless they have an idiot cutting the exhausts who doesn’t know they’re only cutting them for the cats lol thanks for the help so far though; I really appreciate it. I’ve been posting on here for over a year and you’re the the person to even
Attempt to help me

 

They are OBD2 as well.

I dont think I have ever heard of pulling out a 02 or a/f sensor being a diagnostic method to see if it is bad..... Highly doubt it FIXED the issue. It just masked the issue probably, putting the car in some kind of base map or allowing enough exhaust pressure to escape out the hole

I think you just need to rule out "cheaper" to fix possibilities before you go down the rabbit hole.

Get yourself some cheap ramps to roll the car up onto and then a solid wood block to put in front of the rear tires and you should be good.

Can you put the original ECU back in since the replacement did not solve your problem?

Have you replaced the spark plugs? What plugs did you use?

If you remove the section of exhaust with the second cat and see if any big chunks fall out then maybe that's the reason. if nothing...then you have ruled out a cat issue and spent $0 dollars.