when it has no clothes it dries good? How can you tell
Always disconnect the exhaust vent before testing the temperature of the output air. Do not put clothes in the dryer but start a 60 minute timed dry cycle. With your thermometer in back, the temps should cycle between 160 and 120 degrees as the thermostats respond. If the tempsfall off after a few minutes, the coils are likely bad.
If the temps stay good thru the whole cycle, your vent is the issue.
Please reply with the model number inside the door, thanks.