The thermometers built into the grill which is on the mounted to the top of the grill reads 100°F higher than a thermometer inside of the grill.
Heat rises, cheapo thermometers aren’t super accurate.
If you can get access to the probe itself, try the two easy extremes… boiling water and ice water. See how far off they are. They will be off, it just depends by how much.
Of course the important metric when it comes to cooking food isn’t the air temp, but the temp inside the food, which is why an internal thermometer is what most people go by.
Most of the built-in thermostats on barbecue grills are garbage. Before speculating on what might be behind the temperature difference, get a thermometer that is accurate with two probes and measure again.
Thermometers, like most measurement devices, are always accurate until you get two of them. Each device has a specific tolerance (or should, otherwise it’s probably a horrible tolerance), for a grill thermometer this will look like -/+5C/10F. Additionally, everything used to read a measurement needs to be calibrated regularly to ensure proper function, otherwise readings cannot be trusted. For a thermometer, the easily accessible way to calibrate are to use ice water (does it read 0C/32F) and boiling water (does it read 100C/212F). Using these constants will allow you to adjust your thermometer and get a (more) accurate reading.
The thermometer on my last cheap grill worked once and then always said 300°.
Ouch. Sorry to hear that
Could be calibration or hot air rising? Where are you putting the other thermometer in the grill? What happens if you stick them in boiling water?
Heat rises.
The thermometer in the lid is at the top of the thermal stack. The one on the grill near the bottom.
