The heater relay will cut out as soon as i turn on the heater but will not blow the fuse on f150 2009

OXY SENSOR IS SHORTED(was)AF (named by Lexus oddly)
THE FUSE BLOWS IF SHORTED DID IT? 25amps blew up?
AND WHICH RELAY, EXACTLY.
both p1135/55 means , B1S1 and B2S1 both front sensor heaters are dead but gee wiz with relay pulled sure they are. dead.
AF heater relay runs both those 2 front O2 sensors
and if either of the 2 short. fuse marked AF heater fuse blows 25AMPS
The heaters are spec at 5 ohms so each heater uses 3amps 6 total paired, and 25amp fuse does not blow.
but if the 2 heaters B1S1 and B2S1, short the fuse blows.
so bad 02 front sensor. or wires to them melted or cut in half
wires on O2 , with clamps missing allow wires to touch 2000f header pipes(exhaust ) and melt and burn up, and then short.
or even burn open.

or put in the wrong 2 front 02 sensors or used the cheap ones
$20 that are hand wired, and wired up wrong.. (done lots but not wrong yet, I'm careful) I run BOSCH only, and or OEM fit
and easy to pop in, the Bosch web site has the Ecatalog to get the correct ones. from them.

Never heard of a relay problem blowing a fuse. Believe either your heater motor has an internal short, or a short in the wiring to it, like bare hot wire touching metal.
That's a large fuse. Are you sure the heater circuit is the only circuit for that fuse? If it is, makes it easier to find the short. Unplug the heater motor and see if new fuse blows with key on, and heater motor switch on. If fuse don't blow out, probably your heater motor. If fuse blows , could be your resistor pack for the heater or a wire touching metal, it should cause a large spark and a "pop" when it blows fuse.

Did you replace the fuse ? If yes, does it blow after driving ? Sounds like you may have a main power wiring problem, could be shorting to ground. Do you have any codes or an engine light on ?

What else is connect to that circuit if nothing you most likely have a wire touching metal.

blown fuses are an indication of an overload of the circuit ( failing electrical device --relay or module ) or a dead short in the wiring after the fuse. The problem may be in the ECM area. Run the fault codes to determine if electronic problem

Is this fuse also feeding power to something else that is sucking too much power? when it has blown, what else stops working, because there is no power to it? is it dash lights?, the stereo?, any other thing?

Look under the hood at the fuse/relay panel, lift the top off the panel and look inside the lid for the correct location for the a/c-heater fuse and relay. check the fuse if it is ok then replace relay and the problem should go away. Hope this will help you.

It sounds like the heater motor may not be operating. When you switch the heater fan on can you hear the motor running? If not then it may have blown the heater fan fuse, check the fuse's and then try it again. If the fuse is ok then it could be a faulty switch or fan motor.

It is located behind your radio/heater control bezel. Labor time to replace 1.4 hrs

Disconnect the battery and do a continuity check from the heater fuse to ground. It will probably be a short. If so do a physical check of the wire(s) from the heater to the fuse. Look at any place where the wire(s) bend and any place where it's possible for the wire to vibrate against metal even if you have to cut tape and pull it out of a bundle to follow it, retape it later. I have faith in you finding a burned wire, good luck.