The Ram 2500/3500 6.7L Cummins runs a Selective Catalytic Reduction (SCR) system that injects diesel exhaust fluid (DEF) into the exhaust. When the DEF system flags a problem, the truck starts a countdown that ends with a 5 MPH speed limiter until you fix the root cause. Most warnings trace back to a DEF quality sensor, NOx sensor, or DEF heater - not the fluid itself.
Cummins DEF warnings escalate. Ignore them long enough (typically 100-200 miles or one ignition cycle after final warning) and the truck drops to 5 MPH until a dealer or scan tool clears the active fault. Always diagnose before driving long distances.
2013-2018 Ram trucks are notorious for NOx sensor failures - especially the downstream sensor. Common codes P229F, P20EE, P2201. Sensors run $400-$700 each plus labor.
View P20EE Diagnosis →The DEF concentration / quality sensor reads urea levels. When it fails or sees contaminated fluid (water, wrong DEF), it logs P204F. Replacement plus DEF flush is the typical fix.
View P204F Diagnosis →In cold climates DEF freezes below 12°F. The tank heater must thaw it before injection. A failed heater logs P20B9 or P203F and triggers the warning especially after cold starts.
Get a free diagnosis →The DEF injector crystallizes shut from extended idling and short trips. Symptoms include P20E8 (reductant pressure too low) and visible white crust around the injector tip.
Get a free diagnosis →Water-diluted, expired (>2 years old), or off-brand DEF triggers quality sensor faults. Drain, flush, and refill with API-certified DEF. Always store DEF below 86°F.
Get a free diagnosis →Salt and road spray corrode the DEF tank harness connector on 2013+ trucks. Intermittent warnings that come and go often trace to this connector.
Get a free diagnosis →Run a free AI diagnosis tailored to your exact diesel. Get the most likely cause in under 30 seconds.
Run a Free Diagnosis100% free · No signup needed · Powered by NHTSA + AI
Ram TSB 18-009-16 covers DEF heater software updates on 2013-2016 6.7L Cummins. NHTSA Recall 19V-019 addresses 2019 Ram 2500/3500 SCR catalyst replacement under emissions warranty (8 years / 80,000 miles federal, 10 years / 150,000 California). Fiat Chrysler also extended the SCR catalyst warranty on 2013-2018 trucks through customer satisfaction program X37.
If you see a check engine light, these codes most often relate to the issues above. Click any code for full diagnosis steps.
Usually 100-200 miles after the first warning, then the truck enters a countdown. Once it hits zero, speed is limited to 5 MPH until the fault is repaired and cleared.
Only if the warning is a low DEF level message. Quality, heater, NOx, or injector faults require diagnosis with a scan tool - topping off does nothing.
It is illegal under the Clean Air Act, voids the powertrain warranty, and prevents emissions registration in most states. Stick with repairs.
Sensor-level repairs run $500-$900. Tank heater or pump assemblies run $800-$1,500. SCR catalyst replacement at out-of-warranty rates can reach $3,000-$5,000.
P204F is a reductant (DEF) system performance fault - usually a quality or pressure problem. P20EE is an SCR catalyst efficiency fault - usually NOx sensor or aged catalyst.
Yes. Below 12°F DEF freezes. If the tank heater is weak the truck logs P203F or P20B9 on cold starts. The warning often self-clears once the cab warms - but if it returns, replace the heater.