Press the red reset button on the receiver. When the receiver starts back up and gets to the screen that says "Running receiver self check" press "Select". This will enter the receiver into diagnostics mode. You can run multiple tests here, but the one you're mainly concerned with is the Fan test to see if your cooling fan is working. Scroll down to and press select on "advanced tests menu" then select on "Internal Components Tests". Then, scroll down to Fan test, press select, press select on run test to see if it passes. if it does not pass, you need to leave the receiver shut down and get it replaced.
You may need to give it more than 5 minutes. This is NOT a common code. When this is happening you're getting dangerously close to burning out the receiver. Please check area on sides and on top of box. You should have 2-3 inches 'minimum' on all sides. If it keeps happening call 1-800-531-5000 as you might need a replacement.