Rep Power: 0

Bought a 2000 Honda Civic EX (D16Y8) Bone stock that was a salvage/rebuilt car with supposedly a rebuilt engine. Car was fine until I put a downstream heated o2 sensor in (Denso Part) Now my STFT randomly spike to 14+ and hover around 7+ cruising on the highway while at idle it will hang right at 4.6+ and only goes negative while decelerating to around 10-. The LTFT hovers around 3.9+ to 6.7+ while driving. Now I am also getting random misfires when starting cold or hot and sometimes random misfires while driving (P1399).
Car drives mostly alright and seems to have decent acceleration but sometimes bogs down for no apparent reason (It does this regardless of the misfire) Nothing on the car is after market. the resonator is rusted out so the exhaust is kind of loud. I've adjusted the valves within spec and also put a new valve cover gasket due to a small leak. I have replaced the dizzy cap and rotor with a duralast gold part from auto zone due to the old one being worn out. Unplugging the FPR causes to go full on rich (15- to 20- stft but does not stop the misfire.)
I did just buy a set of denso plugs to put in but have not installed them as of yet. Also suddenly get the dreaded P0420 catalyst code today. I have removed the upstream 02 sensor to see if there was a blockage in the exhaust and it actually made the symptoms worse. Tailpipe has plenty of flow and engine doesn't seem like it is being restricted. No visible smoke from the tail pipe either. I removed the timing cover and checked the timing marks on the crank and cam and they line up perfectly. My radiator did blow two week prior but I caught it
before any overheating (lower than 220). Engine temp is fairly stable and comes up within 5 minutes to 186 and peaks at 214 in heavy traffic but normally does not go over 208 (live data readout) temp guage on the dash stays right below the little boat icon right over the squiggly line. I Also have tried cleaning the throttle body and it seemed to help a little as it was also missing the oring on the map (Go Figure) "I fixed that with some rtv for now" Map usually read 8 inHg at idle and fluctuates between 12 to 14 with the ac on while in gear.
Have seen it jump to 29 inHg WOT. I don't have any reason to suspect the head gasket as there is no coolant loss and the overflow res stays pretty consistent about halfway only going a little higher when just shutting the thing off after driving it hard. and very minimal oil loss (seems the oil rings are worn as it does use less than half a quart ever 2k) Once warm it idles fine although alittle high at around 770 RPM and this morning before getting it to pass emissions it was sitting at 1K RPM but I think that is down to the idle adjustment screw being
jacked with by previous owner as he could not keep it running (Stupid aftermarket alarm)

 
Rep Power: 126

I’d probably start with a compression test, then look at that O2 sensor again. Where did you buy it from? Lots of users have purchased Denso sensors from eBay that ended up being fakes.

 
Rep Power: 0

Napa. I don't trust ebay. as for compression test each cylinder is at 178 PSI which is a little low but not bad as it is normal for these things to wear with high milage (I don't think the engine was actually rebuilt , I think it came from the junk yard lol.)

 
Rep Power: 0

I just checked my coolant and oil again after letting it sit for a while and everything is still full "Oil is right below the second hole on the dipstick and coolant is right below the filler neck."

 
Rep Power: 126

Do you know if it’s for sure a Y8 in the car? Could be that they swapped in a Y7 or something else incorrect.

 
Rep Power: 0

It is infact the y8. Engine tag is clearly stamped. And it has the proper head. VTEC solenoid is working as well.

 
Rep Power: 222

Downstream sensor only checks that the cat is working