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Hyundai Elantra downshifts with check engine light

Tiny
DDUGAT
  • MEMBER
  • HYUNDAI
2001 Hyundai Elantra, 76500 miles, runs fine, fluids good, but randomly downshifts from 4th to 3rd on the highway, simultaneous with check engine light illumitaing. Once saw smoke emanating from hood briefly! Could this be entirely the result of an O2 sensor, or maybe a transmission problem?
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Tuesday, July 17th, 2007 AT 9:27 PM

3 Replies

Tiny
2CARPRO JACK
  • MECHANIC
  • 11,533 POSTS
Have the PCM scanned for codes, that will give us a good starting point
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Wednesday, July 18th, 2007 AT 2:33 PM
Tiny
JRM
  • MEMBER
  • 1 POST
I know nothing about cars but-I have a Hyundai Elantra that I thought the transmission was going on, but it was the speed sensor from the engine to the transmission. The car would randomly shift gears when I was driving down the road.
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Monday, August 6th, 2007 AT 10:08 AM
Tiny
2CARPRO JACK
  • MECHANIC
  • 11,533 POSTS
Yep, a bad VSS (vehicle speed Sensor) can give false info to the PCM causing erratic shifts
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Monday, August 6th, 2007 AT 11:41 AM

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