Automatic transmission downshifts at high speeds + check engine light

2003 automatic five speed (original owner)124,585 mi.

1) will not shift to 5th gear (via selectronic)

2) downshifts to 3rd after 12 miles @ >60 mps

3) amber check engine light comes back on @ each time of above refereed to downshift

4) transmission shop unable to diagnose point of failure, wants to replace the speed sensor for $350

5) symptoms repeat each drive and have been present for > month.

6) fluid and filter never replaced

Help, please help : ) - Missy

"2) downshifts to 3rd after 12 miles @ >60 mps "
I am surprised that it works at all now. How did you ever get it to 60 mps (miles per second)? 3600 MPH is well above the ratings for all of the drive line components.

But seriously,
You need to find a good dealer or a good independent transmission shop. There is no reason or excuse for guessing at the problem. What are the diagnostic codes? Are you using some chain transmission place?

Oops, yes, more like >70 mph. spent > 2 hrs. on the free diagnosis, no codes provided. I don’t trust my local dealer. What should I ask for at the next place I try and should I pay for a diagnosis? Should I just get the sensor replaced and take it from there? Thanks a bunch. Fingers crossed. - Missy

I would certainly look for another independantly owned trans shop in your town. Stay away from the chains. If they don’t offer free diagnosis, at least see if the dignosis costs can be deducted from the bill if you have them repair your car, as it appears you will need a repair. I would not let anyone replace a speed sensor for $350 at this point yet. BTW I was thinking the same thing as tardis, if you were traveling at 60 MPS, your transmission would be pretty low on the list of things I would be worried about, I’d be thinking about things like not having a G-suit, not blacking out, possibility of getting a ticket (how many points would that deduct, traveling 3,530 miles over the speed limit?

Fluid and filter change would not correct the problem, right?
No ticket worries, if I see a patrol car I’ll just advance to warp ; )

If the oil was so dirty or the screen/filter was plugged, I wouldn’t think it would shift by itself, I’d be thinking more along the lines that it’d act like it was low on fluid and just not go. But I’m only guessing. Perhaps transman or someone with a knowledge of automatic transmissions will jump in here…

This should be fairly simple. If the check engine light comes on, there should be some sort of code set or some fault that can be detected on a scanner. Are you saying that there are no codes stored when the trans shop scans it?? If the vehicle is not acting up when the trans shop has it then I would drive it until it acts up then without shutting off the motor take it immediately to one of your local auto parts stores and have them scan it with one of their code readers and see if a code pops up. Remember, when it starts acting up and the check engine light comes on, DO NOT shut the motor off. Take it directly to a local auto parts store and have them scan it while the engine is still running. Post the code/s here and we can go from there. If the transmission shop cannot duplicate the problem why do they want to replace the speed sensor?? Thats just throwing parts at it hoping for a cure. Yes, the speed sensor could cause this problem but there are other things which could be doing it also such as a TPS (Throttle Position Sensor).

Heres a snapshot I took on my scanner of a bad TPS a while back. Notice the spikes on the bottom part of the screen. This is the TPS maxing out by itself. My customer would be cruising along on the highway at 70mph when the TPS would max out. This was telling the computer that the driver was flooring the accelerator when she really wasn’t. Her transmission would kick down to 3rd gear and the check engine light would come on.


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Speed sensor for $350 sounds like you need a new shop. The trans need a good diagnosis probably dealer if they are trusty. Other wise tradeway this car. The trans sounds bad to me. That said I took my car 70k to the dealer who told me no codes charged $230 and sent me away. My trans is actually still working a year and a half later but I do have to shut the car off about 2-3 times a month to reset the problem. I drive it everyday.

Transman is on the point IF your local can scan the transmission. It can be a an electrical connection or a whole bunch of other stuff but start with the stuff that is easy.

You are the 1st person who actually seemed like you know what you’re talking about. I had the same issue on my 2004 Hyundai tiburon I changed the TPS and it fixed the problem but my check engine light was telling me it was the oxygen sensor