I have a convection oven that's getting on in years. So the other night when the thermometer I keep inside to monitor temps only went up to 220 degrees, when it was set for 275 degrees, my immediate assumption was that the oven had finally died/failed.
Luckily my sweetie didn't make that assumption. He went out and got new thermometers and discovered the oven was fine, it was the THERMOMETER that had registered the wrong temp / failed. A new thermometer for under a few bucks is much preferable to the price of a new convection oven!
Just wanted to alert everyone else... don't assume it's the oven like me!
And even more so, the sweetie/engineer says 2 thermometers, of DIFFERENT brands, is best. Apparently those thermometers aren't all that perfect either, so if you get 2 of different brands, you're much more likely to get the real temperature. Just figure it's somewhere in the middle between the 2 temps that register. The 'different brands' is important, because each company calibrates differently.
((You know you're a polymer clay addict when you're too cheap to turn on the air conditioning for yourself, but if your clay gets too soft, you will turn it on then.... ))