Subaru Legacy BN/BS (2015-2019) Owners Manual: If the light illuminates constantly

If the light illuminates constantly while driving or does not turn off after the engine starts, an emission control system malfunction has been detected.

You should have your vehicle checked by an authorized SUBARU dealer immediately.

NOTE This light also illuminates when the fuel filler cap is not tightened until it clicks.

If you have recently refueled your vehicle, the cause of the CHECK ENGINE warning light/malfunction indicator light coming on could be a loose or missing fuel filler cap.

Remove the cap and retighten it until it clicks. Make sure nothing is interfering with the sealing of the cap. Tightening the cap will not make the CHECK ENGINE warning light/malfunction indicator light turn off immediately. It may take several driving trips. If the light does not turn off, take your vehicle to your authorized SUBARU dealer immediately.