P204F

P204F means the reductant (DEF/AdBlue) injection system as a whole is failing its performance rationality check - the ECM expects a certain DEF dosing volume per mile based on NOx upstream readings, fuel consumed, and exhaust temperature, and the actual dosed volume is outside that window. Cheapest-first ladder: pull DEF tank level, DEF tank temperature, dosing line pressure (typical 5 to 9 bar when active), and commanded versus actual dosing rate (most systems target 2 to 6 percent of fuel rate by volume); a frozen DEF line, a clogged dosing injector, or a failed transfer pump all show here. Caveat: DEF system fault codes have ramp-down mileage countdowns to 5 mph speed limit on most platforms, so always check the inducement state before sending the truck back to the customer.

Vehicle-specific patterns: 2011 to 2014 Ford 6.7L Powerstroke notoriously fail the DEF heater plus level sender combo unit, Ford TSB 14-0021 and recall 14E04; 2010 to 2016 Mercedes Sprinter OM642/OM651 BlueTEC fail the DEF supply pump after DEF crystallization; 2007 to 2010 Dodge Ram 6.7L Cummins set P204F from a clogged dosing injector cleanable with off-vehicle ultrasonic; 2011 to 2016 Duramax LML throw the code from the well-known DEF heater failure, GM safety recall 14V-252. Estimated repair: $180 to $2,400.