My TDC4 displays the warning "tOPN" even with the probe connected

The following steps should be taken if your TDC4 displays "tOPN" even with the probe connected.

Verifying the continuity of your RTD probe:

Connect a multimeter between the two pins shown in the figure below, using the notched part of the connector for reference.  A resistance between 100 and 150 ohms indicates the probe has proper conductivity.  This does not confirm the accuracy of the probe--only it's operation.

RTD probe connector

Verifying the continuity of the TDC4 RTD receptacle:

Use a small length of wire and a resistor (100-150ohms) to connect the pins as shown in the figure below, using the keyed part of the receptacle for reference.  When the TDC4 is switched on it should read a small, negative value of temperature rather than the "tOPN" warning.  This does not confirm the accuracy of the TDC4 tempeture reading--only proper operation of the RTD receptacle.

TDC RTD receptacle