Looking at the Service Manual SE section, page SE-45: there is a "Fail Safe" mode. If the system detects for 4 seconds that the thermal electric device sensor is an open circuit, it stops all output and enters the system off mode.
It also mentions a 30 C differential between cushion and back, and temperatures higher than 110 C, but that doesn't seem all that likely in only four or five seconds.
Does either the back or cushion begin to warm up before it stops? Since both have a "TED" (SE-77) I'd start by looking at the one that doesn't begin to warm.
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Steve
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