I have experienced some problems with my '01 Sonata auto trans. The
problem is very erratic and comes and goes. When traveling at highway
speed, the trans will downshift from 4th to 3rd gear and the "check
engine" light will glow. Once you pull over and kill the ingition,
restart, and begin travel again all is fine for a while. I have reset the
computer by disconnnecting the battery and this helped for a while.
Recently the problem became more persistent, so I had it coded. The codes
I got were "P1529", and "P154 pending". Has anyone had similar experiences
and can anyone tell me what these codes mean?

 

P1529 is a request for the check engine lamp from the TCM (transmission
control module).

P0154 is a code relating to the front bank upstream oxygen sensor. Check
to see if recall V01 has been done on your vehicle. If not, have it done.
If so, you'll need to replace that oxygen sensor.

But that's not causing the problem you describe. You'll need to have your
car checked by someone who can read the codes in the transmission computer.
Don't disconnect the battery until that has been done!