A "no temperature reading" alarm (open circuit) is the most straightforward thermocouple failure to diagnose. The circuit is broken somewhere. But finding where it's broken takes some systematic checking.
First, Check the Display
The controller shows "---" or another open circuit indicator for that zone. Check the adjacent zones. If multiple zones show open circuit, the problem might be a common power supply or a controller issue. If only one zone is affected, the thermocouple or its wiring is the issue.
Check the Controller End
Disconnect the thermocouple cable from the controller. Connect a known-good thermocouple directly to the controller input. If the reading appears, the controller is fine. If it doesn't, the controller input is faulty. This step eliminates the controller as a cause.
Check the Extension Cable
With the controller end disconnected, measure resistance at the connector on the mold end. Measure across the positive and negative pins. If you have continuity, the extension cable is intact. If you have open circuit, the extension cable is broken. This separates cable issues from sensor issues.
Check the Sensor
If the extension cable is fine, the problem is in the sensor. The internal wire is broken. The sensor must be replaced. If the extension cable is broken, replace or repair it.
The Cable Pinch Test
If the extension cable is intermittent, the pinch test may find the break. With the multimeter connected, gently pinch the cable at intervals. Watch for the resistance to change from continuity to open. When you find the point where it happens, you've located the break.
The Connector Check
Sometimes the connector is the problem. The pins may be pushed back, not making contact. The solder joint may have broken. Check the connector visually and with the multimeter. Replacing the connector is a simple fix.
The Final Step
Once you've identified the problem, repair or replace the affected component. Document the cause for future reference. If the failure was mechanical damage, consider routing the cable differently. If it was corrosion, check the connector seal.
