shifts out of OD, check engine light comes on

[hystat]

99 Sonata automatic 2.4 four cyl. bone stock 207,000KM
I don't have a scan tool here for a couple weeks... scoured the HMAservice site...can't find this specific symptom, but it looks like 3rd is a default "fail safe" gear.. so could be lotsa things..
If I shift to N, shut the engine off, back on and restart the engine I'm good to go again for a day or 2, but with the check engine light still on. Batt disconnect resets the light.
Anyone been there done that recently?
tks

[tranceokido]

try to go to Autozone or a simliar autoparts store, sometimes they do free PCM scans and they can tell you the problem. Once you find out...let us know so we can try to help ya out.

[hystat]

thanks!
I took the batt and air box out- pulled and inspected the in and out speed sensors, cleaned a ton of road salt off the top of the trans and soaked all the connectors....

I drove about 300KM yesterday and it only did it's downshift once, so i think I'm onto something.

[kagemusha]

I may have had a similar problem recently: I was doing about 80 on the freeway and was in 5th gear when it suddenly downshifted on its own into third. Needless to say it caught my attention: hard jerk, made a lot of engine noise, yellow engine light, etc. Like you, I turned the car off and back on and it was fine, but it continued a few more times over the course of a few weeks.

Finally, when it last happened I took it in to the dealership without turning off the engine, they plugged in something that looked like a fancy oversized gameboy under the dash, and diagnosed it right away as a bad throttle chip. I'm sure that's not the right technical term, but it's basically the chip that tells the engine what gear to be in at any given speed. They swapped the chip out in an hour and it was under warranty, so not sure what it might cost if you really do have the same issue. Mechanic said it went into third gear as a failsafe when the chip was malfunctioning.

Not sure if that's the problem but it sounded familiar. Good luck.

[hystat]

after a 600 km trip yesterday i have a pretty good idea of the problem...

pretty sure my lockup clutch is slipping...

I'm told there's an algorithm in the ECM that says "when lockup clutch is on, input rpm must = output rpm" (considering 4th gear ratio of course)
and mine doesnt.... when input doesn't equal oytput (by 5% or so), the ECM detects the slippage and responds by going into default mode (3rd)

I watched the tach and the symptom happens when I'm climbing a gradual incline in 4th... the tach just starts to climb.... then she immediately bangs into 3rd and the check engine light comes on...

soo... I'm going to try new fluid, then probably a bottle of Lucas trans fixer and then probably a one way trip to the dealership in the next few months.

[hystat]

scanned the car- got P0720 Output Speed Sensor malfunction

ok so now I'm confused - is this a bad sensor, or is this a bad sensor reading (i.e. confirming my slipping trans suspicion)... I'm thinking the latter. These codes aren't very descriptive are they... I was expecting sep. codes for signal out of range, input speed not equal to output speed in LU ...no such sophistication huh...I'm used to heavy equipment where the ECM's are much more helpful...

My speedometer has never faltered, which is why I'm thinking it's a slipping issue... I'll have to try driving with the sensor unplugged to confirm that that's what drives the speedo...

I will do some serious connector cleaning and maybe even scratch some plating off the pins... The sensor I assume is a pretty easy check - I'll look for pulses out the signal wire (starting at the ECU) I have a meter that reads frequency.

[hystat]

answering my own post here...X) I ain't slipping 50%...so my problem is electrical... hurray!
from HMA site:
If the output from the output shaft speed sensor is continuously 50% lower than the vehicle speed for 1 second or more while driving in 3rd or 4th gear at a speed of 30 km/h or more, it is judged to be an open circuit or short-circuit in the output shaft speed sensor and diagnostic trouble code No. P0720 is output. If diagnostic trouble code No. P0720 is output four times, the transmission is locked into 3rd gear or 2nd gear as a fail-safe measure.

HMA site rox!

[djnogo]

sounds like the ECU/ECM is confused. try resetting it by pulling the fuse(s) for it in the engine bay for about 5 mins. then let it relearn the correct shift patterns.