Rawze.com: Rawze's ISX Technical Discussion and more

I have a kenworth t800 with isx15 and it has a check engine light and fault code said exhaust 3 temp to hot. What does that mean and how do I fix

Well temp 1 would be the DOC temp sensor, 2 would be the DPF inlet temp sensor, so 3 would be the DPF outlet temp sensor.
I would say either the sensor is bad, or the DPF is full of ash, or possibly cracked. Could also be, although very doubtful, the temp sensor hub unit that all three plug into. it sets on top of the DPF can and you will see all the temp sensor wires going to it. You can test it with a multimeter from pin to pin to make sure none of the circuits are grounding out inside. Quickserve has the testing instructions.
Next:
Do you have INSITE?? If so connect and see what the #3 sensor shows compared to the other temp sensors after the truck has been off overnight and completely cooled down. They should all be relatively the same give or take a degree or two. If it is way off from the others replace the sensor. If it is nearly the same as the others pull and inspect the DPF and DOC.
Good luck hope this helps

So how would I go about changing the sensor? I had my truck hook up at TA for free report with Pm oil change. It was just quick report had inactive 3 sensor fault and also said dpf soot was low. Can i change the sensor myself and how would I do it?

Unplug it, unscrew it, put anti-seize on the threads of the new one. Screw it in and then plug it in. Easy peasy, lemon squeezy!!

Sounds easy ok so do I need to open the metal thing that the dpf filters in or can i reach it the sensor from just under the truck?

You'll be able to remove it without taking anything else off. I find that a carefully clamped pair of vice grips or multigrips does a better job at cracking them than a spanner.

Signature620 is correct, vice grips, or a line wrench, is what I use. A regular open ended wrench will usually slip unless the the sensor was slathered with anti-sieze when installed.