Getting a E05 code on my Mastertemp pentair heater, changed the ignighter ran fine for 2 hours then pool shut off for the day came out to get in the spa turned the heater on and the fault code came on again E05. What next?

I had same issue yesterday in the field. The sensor was bad in my case. After replacement of the Stack flu sensor my stack temp was only reading 80 degrees, a clear indication that the board needed to be replaced as well.
Replaced board and the heater fired fine and the stack temp reading was normal, 300 degrees.
To obtain a stack temp, after the heater fires hold down the pool or spa button until the temp appears in the display. It will time out after 10-15 seconds.

try theflame sensor

This issueis related to the stack flue sensor and it might have a wiring problem or a bad sensor. I recommend you call the Pentair technical support at 800-693-0971and they should be able to help you figure this issue out further. Ihope this helps.

The heater is trying to light but cannot. Either the igniter is broken or you are not getting any gas to the igniter. Check to make sure your gas valves are on.

If you are firing, and then going off and getting an E05, your igniter is to blame. It may be glowing but it's getting weak. The diagnostic goes further but that is usually the fix. with inactivity, the igniter is always to blame. You really need to fire it off weekly to make it reliable.

This is an ignition flame falure code. Could be you don't have gas getting to the pilot, no spark to ignite the pilot or your ignition is not reading the flame. Sorry to say but it's probally time to get the pool guy out to look at it. Good luck.

You are right about the ignitor. Depending on how old your heater is. The ignitor can become weak and flame rectification will be intermittent. You may have to replace the ignitor assembly or at lease check to see that it's not corroded enough to impede gas flow.