Hi, all. I just found this forum, and I have a problem with my wife's 1996 Neon Coupe.
It has the SOHC engine and automatic transmission. She has owned it for two years, and it never gets more than 15 mpg! The inside of the tailpipe is covered with black, sooty crap, and it blows it on your hand if you hold near it. Obviously, it is running way too rich. My mechanic thinks the computer is bad. I don't know the car's past history, but I suspect that its previous owner installed an aftermarket chip to gain power, and it screwed up the computer. Does anyone here have any ideas? It runs fine, but the mileage is awful!

so when was the last time tou changed the fuel filter, air filter, spark plugs and plug wires?

PLugs, wires, and air filter were replaced about four months ago. I have not looked at the fuel filter. If it was clogged, the engine would get too little fuel rather than too much. Right? I don't see missing or stumbling on acceleration like a clogged filter would cause.

Is there any codes?? could be a plugged Cat.

I checked the codes. I get 12, 51, and 42. I think one of them is for a bad oxygen sensor. The mechanic supposedly replaced it some time ago and gas mileage didn't improve.

Well codes 12, 51, and 42 are 12: Direct battery input to PCM was disconnected within the last 50 key-on cycles, 51: Fuel system lean, 42: Fuel pump relay control ckt, Auto shutdown relay control ckt, No ASD relay output voltage at PCM, Fuel level sending unit volts out of range, Fuel level unit No change over miles. I'm thinking codes 51 and 42 are the problem. I'm not sure how to fix those though.

May still be a bad O2 as a lean indication to the PCM would cause it to attempt to richen it up. Have it checked for proper O2 readings and properl switching O2 sensor. They switch as the cause the engine to go rich then lean changing fairly rapidly to assist the CAT be getting it puffs of Oxygen to help catalyze the excess fuel. Sounds as if the O2 is either lazy or defective readings are occuring. Technician should double check after replacement of O2 sensor to verify that the new one is operating correctly.

^ Agreed. Particularly if they were replaced with Bosch sensors instead of the stock (NGK) brand. If a decent scanner is used, it will even tell you which sensor is causing the problem. '96-up Neons used both sensors to calculate long-term fuel trim, so either could be the culprit, although I'd bet on the upstream...

If your '96 has the plastic tank, it doesn't have an external fuel filter. There is a filter attached to the regulator, but it's designed to last the life of the car and then some... You can replace it if you like, but I've never heard of it doing any good for anyone. If you have the metal tank, there is an external filter that should be replaced every 30,000 miles.

Figuring out the cause of the code 42 would also be a good idea. Again, you'll need a good scanner (reads the 4-digit codes) to narrow it down. I'd pretty much rule out either of the ASD relay problems (car shouldn't even start), and probably the fuel pump control circuit (car wouldn't run), which leaves the fuel level sending unit, which is pretty common...

Autozone will usually scan the codes for free. Just write them down and check them at www.troublecodes.net.

Best of luck!