I replaced the rear O2 on my car today and now the engine light is flashing and the car is running rough. Do I need to have the pcm reset with a scan tool? And would the pcm not being reset cause the car to run bad?

 

The rear or downstream O2 sensors should have no effect on how the engine runs, their only purpose is to monitor the cat coverter performance. The light flashing is an indicator of misfire, you may have moved or damaged a plug wire while changing the sensors. If you get the PCM scanned for codes and get a better idea of what to look for.

 

Im gonna get it scanned on monday, but I changed the plugs and wires and I did them one at a time so I don't think i messed the firing order up. i even went back and checked to see if I did and they looked ok to me. I had both 02's unplugged would unplugging them both set the ses light off? When you unplug them i heard it sets ses light on the dash. Also if I somehow did manage to screw up the firing order will they be able to tell which cylinders are missing when they scan it? It doesnt seem like I mixed them up it runs pretty good idling, slightly rough, but not like I mixed up the wires.

 

You can crank the car up, and pull 1 plug wire off at a time, that way you can see which cylinder is missing...just dont get shocked..lol. If you pull a wire off and it makes now difference that tells you theres a prob with that wire/plug....

 

A bad or missing O2 sensor will light the trouble lamp, but the light will be steady and go away once the O2 problem is fixed. The flashing light indicates misfire, and the reason it flashes is to indicate that things can get damaged, In this case the cat will be get overheated by trying to burn off the raw fuel being dumped into the exhaust by the misfiring cylinder. Check the plug wires and make sure none of them got crispified by being too close or touching the exhaust system.

 

I checked them they arent burnt and arent too close. I am going to have it scanned tomorrow to find out which cylinder is missing and then go from there.

 

did you unplug the 02's while it was running. maybe put the old 02's back in and see what happens.

 

No the car wasn't running when they were unplugged. I am getting it scanned for free tomorrow morning, I think it is a misfire, it smells like it is dumping fuel. When they scan it they should be able to tell me which cylinder is missing. That way I know exactly where to look for the problem. Also I guess it could throw a p0300 code for multiple misfire if the wires are swapped somewhere if it does I will just redo the ones I touched (pass side) I have no trouble reaching them. I somehow can reach them all fairly easily from the top without touching anything else. I even left the alternator on. I would go ahead and do it but Running the wires is still a PITA lol. Figure Ill have it scanned to see if it will spare me some time in the long run.

 

ahhh you must have long skinny arms like me. lol.

 

yep lol. Im about to take it to get it scanned Ill let you guys know what i find.