Troubleshooting Thermocouples

By 9 April 2015

Table 5-2 lists possible remedies for a number of different operating problems associated with thermocouples.

Symptom and Possible Cause Possible Remedy
Pilot flame lit but safety control fails to function.
(a) Thermocouple not hot enough to generate current. (a) Wait at least 1 minute for thermocouple to become hot enough.
(b) Drafts deflecting flame away from thermocouple. (b) Eliminate source of draft.
(c) Pilot flame too small or yellow in color due to restricted pilot line or dirt in primary air opening or burner head. (c) Disconnect, clean thoroughly, and reconnect. Change orifice if necessary.
(d) Loose or dirty electrical connections. (d) Disconnect, clean, reconnect, and tighten.
(e) Thermocouple tip too low in pilot flame mounted in bracket. (e) Check installation to make sure thermocouple is properly.
Safety control operates but fails when main burner has been on a short time.
(a) Restriction in pilot or main gas tubing. (a) Eliminate restriction. Provide normal pressure.
(b) Draft-deflecting flame couple. (b) Eliminate draft or baffle.