What if you-all agree to pay that dealer the $300 and it doesn't fix the problem (WHich IMHO would be the result)? Then the next "attempt" must be done at that dealer, maybe at his expense, maybe at yours. If all that is paid in full by warranty, there's still the time and hassle invested. If you "fix" it there, be sure to get the replaced parts.

If possible get the car on home turf, so you can monitor the situation. A failed engine temp sender in a 240 would change things like idle and fuel efficiency, but not to the extent that your Mother's car is suffering.

Good Luck,

Bob

:>)