A CRJ flying at 9,000’ received an EFIS COMP MON caution message:

“Flight Manual directs pilots to slew compass to reliable side. It was apparent neither side was correct with the Captain’s, Mag Compass, and First Officer’s headings all different. We were cleared direct to a fix. Multiple attempts were made to match the headings with only temporary results. The Captain elected to hand fly while the headings mismatched. While the FMS was taking us in a direct line, with the wind shift while hand flying the aircraft ended up 4 miles south of the original ‘direct to’ course. ATC called and asked if we were going direct, I told them we are having heading problems and asked how our heading looked. He told us 10 right and direct when able. On this trip we flew this same aircraft for 9 legs and did not have this problem on any other flight. In the past I have had similar events with speculation that cellphones left on may contribute to the heading problems.

The first Officer made a PA announcement asking everyone to check their cellphones and the flight attendant walked through the cabin. Sure enough, in Row 9 was a phone in standby mode—not airplane mode. The passenger said he didn’t know how to program that so the flight attendant showed him and as soon as the phone was secured, all the avionics worked perfectly.”