Hello everyone! I recently received a 1996 Plymouth Neon as a gift (Mileage = 156,000). It came complete with terrible gas mileage (about 17 mpg) and check engine light! Here's the skinny on my experience so far...

I checked the error code via the dash flash method, and received codes 12 and 21. 21 is, of course, O2 sensor problem. Trusting my more mechanically experienced friend, I replaced the front O2 sensor and cleared the codes. This did not solve the problem, but the check engine light (although still on) would no longer respond to the dash flash trick. So I checked it with an OBD reader at O'Reilly's and got the reading that the rear O2 sensor is reading high. I replaced this sensor also, but the problem remains. I took the car to an exhaust shop yesterday and had them take off the Cat to see if it was plugged. Nope, looks fine. There are no exhaust or vacuum leaks, either. My only guess is that the Cat must simply be not functioning, and the rear O2 sensor is throwing the fail code, causing the car to run in open loop and kill my gas mileage. Maybe the original problem was a bad front O2 sensor, which fouled the Cat, and explains why the CEL no longer responds to the dash flashy trick after replacing the front sensor. It does seem to run better in the intermittant periods between clearing the computer and the CEL returning.

We don't have emissions testing in East Tennessee, so I can't just have them test the exhaust. My local exhaust shop doesn't do that either: the equipment is too expensive. Sooooooooooooo...2 part question:

Firstly, does anyone have a suggestion for somewhere I can get my exhaust tested to see if the Cat is functioning before I buy a new one?

Secondly, does anyone have any other hunches or suggestions as to what else I should check that could possibly be the problem, before slapping a new Cat on?

Thanks a ton for reading this lengthy story and lending me your expertise!!

 

the veterns here could tell you about the 1/4 ohm resistor trick to fool the pcm into thinking the cat was good, but where I live, cat or no cat doesn't matter.

 

Same here, Cat doesn't matter as far as the Law is concerned. But I'd be hesitant to try such a trick if it involves chopping up the wires on my new rear O2 sensor. I's hate to mess something up, and end up having to buy yet another O2 sensor just to get back to where I started...ya know?

 

I too have a problem related to yours. I replaced both sensors about a year ago everything was fine for awhile. Now the dash light comes on and code 21 is stored. It is really strange. It will stay on for weeks at a time and all of a sudden when I start the car it will be off however after driving a few miles and the car warms up the light comes back on. When I shut it down and restart it will be off again but travel a few miles and it comes on again. Eventually it will stay on for weeks again and then go through the same cycle again. It does seem to run better when the ligh is off and the gas milage is not all that bad about 32mpg even with the light on... Anybody got any ideas to what I might look for. And could it be the fuel ingectors that is causing some of the problems. As far as I know they are still origanl 134,000 miles on it...

 

clogged EGR tube?

 

-------------Update (sortof)----------------

Hey guys, what's up. Well, I've been busy with school and haven't messed with the car in a while, but I did the key-dance yesterday and lo and behold a couple new codes. I got:

12: Battery disconnected in last 50 key cycles
51: Engine running lean
42: Fuel Pump Relay/ Auto-shut down relay short circuited or bad
21: The old O2 problem

Interestingly, I haven't disconnected the battery...? Nonetheless, I bought a new relay for $5 and stuck it in there. Engine ran with no codes for a while, but code 51 has come back. On the bright side, I appear to have fixed my O2 sensor error, somehow.

I know the usual causes of a lean ratio, but was wondering if you guys had advice for what order to tackle them in, or if the combination of codes tipped anyone off to a possible cause. In response to Sabin, a clogged EGR valve is still a possible culprit, right? Can it be cleaned (and how), or must it be replaced?

I'm thinking of tackling the fuel filter next...I have no idea when (or if) it was changed anytime in the last 155,000 miles...

Thanks again for all your help, gang

 

code 12 - don't worry bout that
51 - eh...it's a long shot, but the clogged EGR tube could be a possibility, along w/ clogged injectors. run some injector cleaner through it first.

the EGR, if you want to, the tube that runs from the head to the intake manifold, 8mm bolts hold both sides on. you can clean it w/ pipe cleaners and any degreaser. won't harm anything. What the EGR does is it takes your exhaust gas, which also has unburnt fuel in it and puts it back into the intake to burn again. It also lowers combustion chamber temperature that way and creates a little richer mixture.

 

Well what do you know? My 1995 Neon was a gift too. It had 156K miles on it and I just finished rebuilding the thing - which was why it was given to me.....

Anyway, by your post, you live in Knoxville, TN which puts you really close to North Carolina that requires an emissions test for all autos. The charge is reasonable:
http://www.ncdot.org/dmv/vehicle_ser...tion/fees.html

and you can find the nearest one at:
https://apps.dot.state.nc.us/dmv/emissions

Just think, on the way there and back, you can gamble.....

 

egr valve is really easy to change for some detailed info use a haynes manual if you dont know what to do...just a suggestion makes job alot easier lol...but that will stop your car from throwing the code