I have a Trane Furnace. 2 years old. The Error Flash Code blinks 4 times-Open Temp Limit Switch. How do I fix this?

The limits are heat switches, if the furnace temp. goes over the set limit of the switch the switch open stopping current flow for safety. The limits will need to be
Tested with ohm meter when not hot limits should be closed, if all limits test good
A faulty control board may be at fault.(good limit should have a reading on meter)
A open limit will have no reading on meter. To locate limits turn power off and gas
To your furnace ,remove upper and lower panels on the lower panel there should
Be drawings of all componets and wiring symbols also look for limits on the drawings this will help you locate the limits for testing, also make sure filters are clean if limits test good and filters are clean ,control board may be at fault.