carnate69

2014 C7 z51 - 50k miles. Had a misfire a while back, burned up the cat in the process (had a p0420 code after my misfire code), replaced all new plugs / wires, etc. to get rid of misfires with OEM, car ran great but had the p0420 code, so bought a new cat. I took it apart, replaced the drivers side cat with a new OEM GM cat, replaced front and rear o2 sensors (just to be safe, 50k miles on them) with NTK sensors from rock auto. I did replace the manifold gasket with an OEM one.

The car runs fine for about 1-2 minutes, then once closed loop hits, it runs TERRIBLE. It bogs, it won't rev up. It barely wants to idle, it stumbles. It's running super lean and the live scan data says it's adding a lot of fuel (15% or more at idle). When I installed the new cat, the rear OEM exhaust didn't really want to line up great, so I sort of muscled it in there so I'm thinking I could have an exhaust leak pre #1 sensor? It all looks tight, can the gasket go in a different way? No CEL codes at all appear when it's running terrible either.

I know an exhaust leak can confuse the sensors, but is it really that NTK wideband O2 sensors junk on these cars and I don't know it?

I didn't disconnect the battery when replacing the o2's, but have since disconnected it for hours to just be sure the computer is clear of old data.

My plan is to either take it all apart, and put it back together and make sure it's all straight and tight against the header, and install the old o2 sensors while im at it.

The new OEM cat had a dot of welding slag in the threads of the 1st O2 sensor, which made tightening the 02 sensor difficult even after I tried to get the piece of welding slag out of the threads. I don't think this would be causing the leaking issue, but thought I would throw it in there. I'm sure it ate up some of the o2 sensor threads.

I need to put it back on the lift and check for a leak, but thought I'd check here, the NTK sensors are my biggest question for now.

Sensors I used:

NTK 21579 Oxygen (O2) Sensor 1
NTK 21074 Oxygen (O2) Sensor 2

carnate69

Update, may have fixed it - when I put the cat on you have to use a socket that isn't deep well, but isn't a shorty for this one nut, an in between socket size is required. I tightened the exhaust with a flex head ratchet because it fit the best and I didn't get the nut tight enough. I put the middle-sized 15mm socket on the flange nut, and it tightened right up. So far, thinking that it was loose causing an exhaust leak which caused the pre-cat o2 sensor to go nuts, causing the car to run like crap.

carnate69

Not fixed - at a cold start up this morning and the car is running what feels like super rich, won't idle steady, bogs, chugs if I hit the gas, backfires if I blip the throttle. Any ideas?

9sec

are you sure you didnt swap the pre cat and post cat plugs when reinstalling?

carnate69

It looks like we solved the problem, and it possibly goes back to GM quality control. We took the cat back off and realized that not only did the o2 port have welding slag overspray on them, but the face of the cat also had welding slag metal dots all over it, so the cat would have never been able to fit tight against the exhaust manifold. When we took it down, we realized the manifold was dimpled from tightening it up against it with the slag dots. We shaved off the slag welding metal and replaced the o2 sensors with the old OEM ones, and all is good. So it was either NTK sensors (Which were in the right places!) were hateful with the car, or the cat couldn't seal correctly with the 3 slag dots pushing it up and couldn't be tight, or both.

Avanti

I can only offer some general advice from years of sometimes using "superior" aftermarket parts that caused issues. If all else fails, try OEM parts for whatever you've exchanged for others. This never failed to correct problems I had. All the best.

ShawnZR-1

I can almost guarantee it was the O2 sensors. The computer hates aftermarket O2 sensors, I've seen it multiple times. Always go OEM with them.

carnate69

Car has been running fine for ~100 miles with new OEM cat and original 48k mile O2 sensors. Readiness codes are cleared and OK and air fuel looks correct on the live data.