6890 FID erratic makeup gas flow

One of our 6890 GC's shows a continually varying (in one minute the reading can vary up or down 20 ml/min) flow for the makeup gas. I shut down all the heated zones and turned the nitrogen flow off at the cylinder and the display on the GC still showed a flow present which of course was drifting up and down.  All the other gas flows look normal. Any ideas on what could be wrong.

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