Hyundai speedometer

I sold my 2000 elantra to my niece last month and already she is having
problems.
The check engine light comes on steady and now the speedomoter does not
work.
She tells me that she got the oil changed last month and they told her
that she may have transmission problems. she thought that they were
just trying to sell her something that she did not need.
This car is 6 years old and had less than 50k on it.
Any ideas? has anybody experienced this before?

 

As for the CEL, make sure she's screwing the gas cap on until it clicks
several times. It's a common issue that causes a CEL.

 

The lamp and the inop speedo could both be caused by a broken speedometer
cable.