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Hi all, signing up here trying to help a friend get her somewhat abused 02 Civic running better. It had some major obvious problems with the suspension and oil consumption and leakage but also throwing engine codes relating to the downstream o2 sensor. Here is what I did. With all the labor plus waiting for parts, returning bad ones and many trips to Oreilly it took weeks. Annie has sunk a lot of money into this and so have I.

Replace plugs and coils

replace lower ball joints, brake pads and lower control arms both sides

replace O2 sensors

replace pcv valve

new valve cover gasket & seals and new cam plug

at this point, after changing the valve cover gasket the car is almost undriveable. It is sputtering, erratic idle speed, lurches on acceleration and stalls. Strong exhaust smell in cabin and noisy exhaust system and no engine code! The only other thing I touched in the process of changing the valve cover gasket was remove the egr valve to replace the gasket and have access to the camshaft plug (a common source of oil leakage), and move things a bit to get the cover out.

Replaced egr valve, since that was the only thing I had removed

still same problems

replace plug again (they were fouled)

still same problems

replaced MAP sensor and cleaned intake air temp sensor

still same problems

I was very careful in my work and didn’t miss any connectors, etc. All bolts with mfr torque specs are torqued to spec. I looked and looked for anything I could find visually. No disconnected vacuum hoses, no disconnected wires etc. Nothing. After a lot of money and effort I am at a loss for what to do now and the shop charges $200 just to look at it!

thanks for any suggestions. seems like i can't get a fault code when i really need one!

 
Rep Power: 367

no engine codes at all?
Also, honda no like aftermarket electronics, including alternator. Always get denso or NTK.

By chance, are you not able to connect to the ECU? If so, check alternator bolts. If they are loose, they fry the ECU.

 
Rep Power: 222

Guessing you probably used aftermarket o2 sensors. And if you have a lot of exhaust noise and smell, look for leaks in the system.

 
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oop dp

 
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yes. i did use a denso for the downstream but it didnt help and i returned it. we did some more investigating. i unhooked both O2 sensors and the car runs much better but smells pretty bad so we made a smoke machine with a cigar and a pump. it smoked that cigar right up! found no vacuum leaks up top but did find the exhaust manifold leaking at the block and also at the junction with the pipe out. at this point i am wondering if the cat is bad too (from the smell). the OEM manifold is $250 but the OEM cat for the EX is crazy expensive. i will do one more test and try to drive it with only the upstream sensor and then run it with only the downstream one and take notes.
here's a couple of questions:
1. can an exhaust leak at the manifold cause a faulty O2 sensor reading bad enough to make it run so terribly by itself or is it also the cat?
2. the price range of the cat section of the exaust is crazy wide for aftermarket ones like anywhere from $150 to $1500 but if it's more than the price of the manifold section forget it. is it worth a try with the cheapos or should i sell the whole car for parts? it runs OK without the sensors unplugged but would poison anyone in the cab!

 
Rep Power: 367

cat is clogged if it runs better when upstream is unhooked.
do you have smog test where you live?
if not, get the cheaper one for now (you could either have to replace more often or it could not work well, always a risk)

 
Rep Power: 222

Downstream only checks if the cat works.
Upstream is what adjusts the fueling. Get the proper denso one.
if the cat is clogged, temporarily removing the upper o2 gives the exhaust a place to escape and the car may run better

 
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that would explain a lot, especially the failed exhaust manifold. another thing i forgot to mention is that when it's running and you remove the oil filler cap there is a lot of pressure blowing out.

our emission testing here ended in 2020

 
Rep Power: 367

clogged cat is simply a massive constipation... You don't want to be the engine there...
without emission testing, you can also choose a high flow cat and have an exhaust shop weld it in to your current exhaust.
ah, if you get an engine code, read about defouler. You can find those in amazon.
You in TN, I suppose? same timing for emissions as me.

 
Rep Power: 222

could be a just a ton of blow by, how many miles on the engine? How nasty does it look in the valve cover?

 
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so on and on it goes. i could have quit here but just because i have time and like puzzles continuing on. i decided on the cheapo cat without replacing the manifold. at least that would tell if this was the problem ($100 instead of $200 just to have somebody look at it). when the part came i tried to install it and the donut flange didn't fit! since it was cheap i decided to reuse the old flange section and graft it onto the new cat section. now i find the two pipes are slightly different OD so i cut a piece of the old one and cut a slit down it and slipped it over the new one where the clamp is and used a lot of exhaust grease and some jbweld putty around the edges. the car runs now but the exhaust is leaking at both donut flanges but my graft is good. i am still concerned the manifold is cracked and know that is leaky. i think i will just replace the donut seals and spring bolts as the new spring bolts that came with the cat are also the wrong size.

 
Rep Power: 222

Buy once cry once

 
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well i wouldn't have even tried with the $1500 OEM and it works! drove the car today. the new pipe is pingy but sealed and no codes! if it were my car and i was going to keep it i would do the mufflers too. i think it just needed to breathe

Colin, the car has 170K. the inside of the valve cover looked like 170k as far as i can tell but no crusty buildup, just long term hot oil stains.

 
Rep Power: 367

good to hear it worked!
reminder that the cat might not be as long lived as a regular priced one.