Q: Will not go faster than 35 mph, emission light on also on 2005 Mercury Mariner

Rookie cbe0621eac06868b3efe0d8d1d3611e23c60d3114864ea2ec19a68cfbd3eebab
The emission light has been on for a while, then one day, was at highway speed, slowed to 35 mph an would not go faster.
(1) Answer
have the cat converter check for back pressure failure and have the fuel pressure checked.

this is why it is an issue with leaving on the check engine light for a long period of time. we always recommend repairing the car when the light comes on as more damage may result not attending to it at the time of failure.