Cycle the ignition key three times from off-run, off-run, off-run. Then watch digital odometer display for codes. It will say "done" when all codes have been displayed.
Engine idling in park.
Check engine light, see related question below.
go to autozone to ask to check your codes
http://www.obd-codes.com/
The only proper way is with a scan tool.
try disconnecting battery for 15 minutes
Transmission, maybe. Check engine light
catalytic converter failure.
How do I check my transmission fluid on my 2008 dodge grand caravan ... to service a 2014 Dodge caravan w/c van that doesn't have a dipstick to check fluid
It would be very rare for a P1684 code to turn the light on.
It might be that your pcv valve is plugged or the egr valve is sticking.
you need a scanner.there's many cheap "[obd2] scanners out there. or you can go to autozone and they scan it for free.