I don't recommend those analog oven thermometers. I've seen them 50 degrees off brand new. Digital only if you want accurate readings. Also, checking oven temperature needs to be done by getting the average of high and low temps after 3 or more heat cycles as the oven tries to maintain the set point. 350 is the standard test setpoint temperature. Doing it the way you describe here does not take into account that many ovens use the broil during preheat to preheat faster, plus your probe is in the center while the oven temp probe is upper rear and left or right side....., again, near the broil element. BTW, in 13 years of repairing appliances, I have only had to replace one temperature probe and it was a wiring issue, not the probe itself. It's the control board that fails 99.9% of the time. When an ignitor fails, it often will not heat at all, or the broil is the only heat source during preheat so it takes a long time to heat up. If it actually reaches the setpoint, the the broil drops out and the oven cools back down since the bake ignitor is not working. You may also smell a mixture of gas and a sour metallic smell from improper ignition.

thanks for info