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2004 Dodge Neon SXT transmission not changing gears

Howdy y’all. Let me say first, I know very, very little about cars. But it all started when I was driving down the road and my car was bucking around 40-45 mph, it stayed that way up until I put some gas in the car the next day and the car started shaking violently when I turned it on. Ever since then my car has been not able to change gears. Reverse works normally however. Now the speedo doesn’t work at all and somehow the has gauge will fall or rise depending on whether I’m stopping or going. So I ran to AutoZone and got a diagnostic ran on my car. It gave me codes P0700, P0731, and P0720. I kept calling around and got horrific quotes around a thousand bucks and mechanics saying my transmission was out. However, I turned it over to a close friend who could work on it for free and has some mechanic experience. The first thing we did was replaced both the output and input speed sensors, which didn’t fix it. Then I got a new transmission control module, which for my car I assume it is the TCM and solenoid. Still. That didn’t fix the problem. Those were some common fixes I found online for neons. But I have no idea what else it could be. I’m posting this as a last resort, hoping somebody out there has an idea what may be going on. Any advice is greatly appreciated. Thanks.

Getting a quote over the phone without the mechanic seeing the car, is totally useless. Would you ask a doctor to do something similar?

Take the car (physically) to a good mechanic and get a quote. Don’t waste money buying anymore parts without a diagnostic.

^^ Agree. Don’t throw money at a problem unless you know exactly what the problem is. Find a good mechanic and have it properly diagnosed. It may not be as bad as you fear (then again, it might be worse). There’s no way to know without a proper diagnosis. Just because you have a code or ten, doesn’t mean that all the systems indicated are bad.

Take BillRussell’s advice. Continuing to just “throw” parts that don’t work at a problem can soon add up to quite a bit of money wasted.

If your still not getting an output speed sensor signal (P0720) inspect for broken wires near the output speed sensor connector. Test the wiring between the Transmission control module and the output speed sensor for continuity.

Have you checked the transmission fluid level? If so, does the fluid look like it is still in good condition?