Please select one of the following items to troubleshoot: Low or erratic readings are caused from loose or dirty connections.
Check all connections make sure they are clean and tight.
To make a quick field check on the thermocouple - disconnect it from the gauge and use an ohmmeter.
It should measure around 2.5 ohms across the leads. this will vary slightly depending on the temperature difference between the cold and hot end of the thermocouple.
If it is higher like around 10 ohms or more the thermocouple has failed or is failing and should be replaced.
These are hard to measure accurately so make sure your ohmmeter is zero out before testing.