The Federal Aviation Administration (FAA) conducted an internal audit in December of the Boeing 737 Max and found wiring issues could potentially cause a “catastrophic” short circuit at the rear of the plane and lead to a crash, a senior engineer at Boeing and three people familiar with the matter told The New York Times.

Boeing is examining if two wiring harnesses at the rear of the plane are too close together that would result in an electric short that would cause the plane’s tail to malfunction in flight, said one of the sources.

If Boeing decides to fix the wiring problem, it would mean that more than 800 Max jets would have to see wiring reconfiguration.

It was unclear, however, if simple also means cheap, and some have speculated that a full-blown recall could cost tens of billions.

The FAA said the wiring harnesses are too close together, located at the rear of the plane, would cause the motors that control the stabilizer, a horizontal fin on a plane’s tail, to malfunction (short circuit) and could lead to a potentially “catastrophic” crash.