98d16y7

Hey y'all. I recently got a 98 honda civic dx with the 1.6l, 5 speed. The car has a header and cold air intake (installed by previous owner). It had a plug in the downstream 02 spot and when i went to install the downstream i ordered i noticed the upstream was the exact same wire length as the downstream i was installing, from what i had seen the upstream should have been shorter. Anyways, i installed the downstream, cleared codes and took it down the road. Same problem. Hopping and jerking and the throttle was super touchy. Unplugged the "upstream sensor" and bam, no more bunny hopping, touchy throttle, etc. Ordered an upstream sensor and drove it to work. All was fine, or so i thought, until i was on my way to work (about a 45 minute drive) and all of a sudden it missed. Thought okay maybe it was something i did and didnt realize it. A good ways on down the road it happened again and i realized that the cel had flashed off n back on (on because of no upstream 02). Did it one more time before i got to work. Put the new 02 in when it arrived, cleared codes and took down the road a few miles. All seemed well. Took it to work the next day. Same thing. It randomly misses and ive noticed its around 3k rpm, usually between 280p to 3200. Has done it in 4th or 5th gear. I replaced the plugs (ngk's), wires, distributor cap and rotor button. Still. Misses 😠 the guy i got it from had it parked for a year n a half or so. I drove it home n drained the gas out, put fresh in and a bottle of seafoam. I have not changed the fuel filter yet but i dont see that being the problem. Most of the time, not all the time, when it misses it will throw a p0505 code. Also, after a few days of driving with no 02 codes, it will throw a p0135, heater circuit, bank 1 and 2 times it has thrown a p0420. Both occur after i drive it for a few minutes. I can let it sit and idle for 5 minutes and nothing but once i head down the road. Bam. Also noticed if i drive it harder it doesnt throw codes as quick. I have been told the 0420 is because it has a header but i never had that code before. Anyone had this problem or know whats going on? I have searched and searched but not found anything. Also, the iac wasnt working properly when he started driving the car again but he cleaned it twice and it seems to idle just fine now (usually around 1000 rpm). Any help or thoughts would be greatly appreciated!

mk378

Getting both O2 heater codes simultaneously on an OBDII the first thing to consider is lack of power to the sensors due to the well known Fuse 15 Problem.

The stock DX has the cat on the front of the engine, so both of the harness sensor plugs are near the front of the engine. You will need to extend them both to use a header and cat where the sensors are under the car. Code P0420 means that the reading from the downstream sensor suggests the cat is not actually converting anything. So a few questions; Is there actually a cat? Or did the previous owner fit a straight pipe, or a hollowed out cat? Is the downstream sensor installed after the cat?

When the engine is fully warmed up and the upstream O2 sensor is working, the ECU will transition into "closed loop" operation, which leans out the mixture for better mpg and lower emissions. This also makes runnability problems more apparent.

I would start by checking the spark timing. Also check fuel pressure and inspect the spark plugs to see if it is running rich or lean, and if all the cylinders are about the same.

98d16y7

There is no cat. The header goes to a straight pipe. The downstream is 6-8 inches back into that straight pipe. The spark plugs are fine. I had put auto lites in it for about a week and my mechanic said i needed to run denso or nkg's so i bought the nkgs and swapped them out and the auto lights looked fine. There is no codes being thrown for the down stream sensor. Only the upstream. The p0420 has only thrown 2xs out of 20-25 some runs.

98d16y7

Those pics were before it was throwing the p0135