LIMP mode/ECU

Tiny
DUTCHGIRL
  • MEMBER
  • 2000 SUBARU LEGACY
  • 2.5L
  • 4 CYL
  • AUTOMATIC
  • 215,979 MILES
I had the transmission rebuilt about two years ago. I got a flat tire put the spare on and after that it seems the transmission was messing up again. I was told that the car could be in limp mode because it is an all wheel drive vehicle and putting a spare on the car caused it to go into limp mode. I got a diagnostic reader, which also reach transmission codes, because the automatic transmission oil temperature light has been flashing and I cannot tell what the code is by the flashing light (short flash/long flash). When I go to use the code reader for the transmission it says communication information fault it is like it is not communicating with the car's computer (It will read OBDII codes fine). Is there a relay or fuse I should try before getting another ECU? If I get another ECU will I have to have it reprogrammed for my car? Also, how can I tell if it is in limp mode and how do you get a car out of limp mode? Thank you very much any help is greatly appreciated!
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Tuesday, April 10th, 2018 AT 5:59 PM

1 Reply

Tiny
STEVE W.
  • MECHANIC
  • 13,947 POSTS
Subaru's are picky when it comes to scan tools. I doubt the ECU or TCU is the problem as it is pulling codes okay. More like that scan tool just does not like that car. Happens all the time, one of the reasons why it costs a ton to operate a shop, you need to have four or five tools to talk to everything.

That said, yes installing a spare will put the transmission into limp mode if it is any different in diameter than the other three tires on the car. Next, to get it out of limp mode should simply take replacing the spare with the correct sized tire. However, the flashing temperature light will keep it in limp mode as well. For that you need to use a meter to test the sensor as well as verify that the transmission connector is clean and secure. As the transmission was out it could be a loose or corroded connection or just a bad temperature sensor.
My first thing would be to have a different scan tool attached and see what that shows first. Then see if the sensor is showing an odd reading, like 200 degrees when the car was just started and cold or -40 when it is warmed up. Then go from there.
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Wednesday, April 11th, 2018 AT 1:12 AM

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