I replaced the thermocouple, and now the kiln no longer fires accurately.
Did you replace the thermocouple with the same type as the old one? (Types K, R, and S are not interchangeable unless the controller is reset in Options for the new thermocouple.)
Are all the thermocouple wire connections tight? (A thermocouple screw can be tight, but the thermocouple wire under the screw can be broken and barely making contact with the screw.)
Are the wires connected to the correct terminals? (It is possible to fire the controller even with reversed thermocouple wires, but the temperature will be off.)
Is the thermocouple touching the stainless steel kiln jacket?
Is the thermocouple pushed far enough into the firing chamber?