Dyer dryer does not dry in timely mannor. takes up to 4 hrs to dry one load

the heat sensor is bad

Make sure the lint trap in the door is clean. It has a plastic cover that opens. Disconnect the venting at the rear and run a load, if it works you have a restriction in the hose after the dryer or in the wall past the hose.

my brother repairs dryers and he thinks the thermostat has gone

Check vent path from lint filter all the way to the exhaust vent to make sure it is clear. If the dryer has a wet/dry sensor it could be malfunctioning. Try a manual cycle to see if the problem occurs then.

If there's heat inside the drum the heater coils are ok. Check the exhaust vent, make sure it's not kinked or squished behind the machine and that the outside vent is not clogged with lint. You should feel air blowing freely to the outside.
If there is no heat, first check for 240V at the outlet. If you have 240V, it could be the element, the hi-limit thermostat, the thermal cut-off, the timer, the motor or a wiring issue (loose or shorted)
Good luck!.

Try this and let me know what happens.
TIP OF THE DAY
To reduce the suction draft and drying time your drier and save some cash, remove the lint catcher and take it to the sink. Run water into it, if there is excessive pooling of the water (water not running straight through) then fabric softener is fouling the lint screen. Use a small brush and dish detergent to clean it and then shake off all the water and dry the screen. Remember this tip on future driers and you will make the dryer blower motor last much longer.

:?) DRRXR

If it's new take your receipt and go to where you bought it and ask them to look into the problem.

try drying load with vent tube disconnected may get warm and dusty but if works ok recheck vent completely also be sure vent is not being flattened when dryer is pushed back to wall also remove lower panel(clips along top edge each corner about 3-4 inches in)remove blower housing 2-3 screws and clean housing

Did you find what it was?

Check the dryer venting from the dryer to where it exits the house. It is most likely coated with lint and needs to be cleaned out.