FG 1.020

And therefore you have a higher final gravity measurement like you state in your original post, unless you compensated and added water to get to your needed level of 5 gallons. Or am I missing your point?

 

I've had the same problem taking FG readings with CO2 trapped....but I don't think that is the central problem here. This was supposed to be a Pale Ale with limited bitterness to begin with and by overshooting your OG/missing volumes, you essentially have a stronger, maltier beer that will be lacking the bitterness ratio you originally intended. Dryhopping might help some, but with that OG/FG, you might be dissappointed. Cheers

 

@Eriktheipaman , maybe I missed it in this thread but what was your mash temperature?

Cheers!

 

This could be the culprit, had the same issue.
Thermometers do fall out of calibration, especially as you said it has been some time since your last brew.

 

I assume by turkey fryer thermometer, it's one of the metal ones with a long probe and a dial-style gauge on top. I find that those get knocked around quite a bit and should be calibrated from time to time. Use your glass thermometer to calibrate with some boiling water. You might be surprised by how much they drift.

 

@Eriktheipaman , I don't think there is much issue other than you're yeast just crapping out early. Any fermentation off flavors? 1.071 -> 1.020 puts that Cal ale at 72% attenuation. That's not terrible, especially considering almost 10% of your grist is flaked oats which is carrying over some unfermentables and proteins that is likely keeping that gravity more dense.

If you're concerned about this in the future, you can always keep ~500 ml of the batch separate and do forced fermentation on stir plate at higher temps to see the max attenuation level.