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P1529/P0720 Error Codes Generated: What should I do?

(2002 Hyundai Sonata)
in Sunnyvale, CA on August 15, 2014
I was cruising at 65 mph set on cruise control after only ten minutes when all of a sudden, it felt like my front end dropped and I noticed that the RPMs were running in 3rd gear. My gear slipped. Since I was on a bridge, I slowed down to 55 and pulled off immediately.

I turned off the car then turned it back on. No problems all the way back home (1 hour drive freeway)

This morning, after driving on the freeway for about 45 minutes, the gear slipped again when I was going about 55 mph with the RPMs revving in 3rd gear.

I am taking my car for an automatic transmission flush this afternoon but was wondering about the P-error codes. I noticed it said output but should I also replace the input speed sensor as well?

No smoke expelled, no burning smell, no loss of power before the gear slipping, and no clunking noises.

Thanks so much!
Displaying 2 answers & 2 comments
Popular Answer
on August 15, 2014
If you are could be the output shaft speed sensor. Otherwise, see answer #1.
on August 15, 2014
The P0720 code indicates the output speed sensor. I will purchase it and replace and see if that alleviates the issue. Thank you.
on August 15, 2014
Flush won't help. Probably need another transmission.
on August 15, 2014
I was in fear of that :(
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