why does my malfunction indicator light stay on in 2012 Frontier?

What malfunction is it indicating..... 2012-- -I hope you still have your Owner's Manual? Look up what the book says about the particular warning light that is on.

P.S. then get back to me.

P.P.S. - checked the fuel cap,etc. says nothing. There is a whole bunch of possible etc. to check.

yes, it indicates that is the light is steady - an emission control system malfunction has been detected. Check the fuel-filler cap, this has been done - no other message appears - just the MIL light - steady.

so I kind of wonder what car owner uses the terms -"malfunction indicator light" &" MIL" in the proper context?.... The light is merely indicating that an emission standard has been exceeded( MAYBE*). It could be anything from a rotted out/broken/dirty part from intake right through to exhaust or just an aged sensor anywhere from a to z. A scan tool is needed to read a code AND THAT IS JUST THE START OF THE JOURNEY. So get code read (apparently some places will do this for free and ask for code(s) then DON"T just buy a sensor(s) and ASSUME that anything has actually been fixed.....*if the sensor is fine then something is WRONG; and, the sensor did it's job. The sensor can be NO GOOD and emissions are fine...... and so the journey begins.....You never even mentioned how the car was running; so again, what etc. did you aready check?

I was using the terms found in the manual - I'm obviously not familiar with car repairs and the terminology that apparently goes with it.

no problemo - I was merely curious.My answer is given in all honesty regardless of whom you are. read my answer and start with getting the scan.