Might just be a faulty temp sensor???

Is the display you are talking about the one on the heater itself, or another one, such as an easytouch, or intellitouch system?

Thanks for the responses! The display is on the easytouch (maybe intellitouch - I'm at work, so I can't check) remote pad. The Heater itself is displaying nothing at all. That's why we checked the breakers - to see if it was getting power. All of our equipment is Pentair. Thanks for any help.

The controller tells the heater to come on when it senses that it needs heat. So if you have a faulty temp sensor saying the water temp is 204 degrees, and you are trying to heat it to something less then 204 degrees, it wont come on.
Need to replace temp sensor, this is an easy do it your self project, or it may still be under warranty.

Thanks Just-a-pb and landa_mac- it's still under warranty with Pentair, so I don't think we'll touch it ourselves. I'd really rather have our local repair guy work on it, since we've been dealing with him so many years, but we don't want to void our warranty. Those things are so expensive - but they do a great job of heating the pool!

My pentair MT 400 fires up but the service heater light comes on and the heater shuts down after 15 to 20 seconds. The AGS sensor light light comes on. I have replaced the gas valve and the gas co. Has replaced both the regulator and the meter. I only get about 8.5 wc of gas pressure (static) and it drops to 0 at ignition. The heater has worked fine for 6 years. Ideas?