When you drive a Class A motorhome powered by a Cummins X12 engine, your comfort, efficiency, and compliance all rely heavily on the emissions system doing its job. One component that often flies under the radar—but plays a critical role—is the DEF (Diesel Exhaust Fluid) head unit. The Tiffin Motorhomes Cummins X12 DEF Head Unit (OEM 4310-18-39) ensures that sensors, control logic, and dosing all stay aligned. A failing or miscalibrated unit can lead to warning lights, engine derates, or emissions faults that spoil the open road.
-
Monitors DEF fluid level, temperature, and urea concentration, providing accurate data to your vehicle’s ECU
-
Regulates precise dosing of DEF into the exhaust stream so the SCR (Selective Catalytic Reduction) system can effectively reduce NOx emissions
-
Built to match the OEM 4310-18-39 specification, enabling plug-and-play compatibility with Tiffin X12 setups
-
Helps prevent fault codes, warning indicators, or engine derates caused by sensor drift or inaccurate readings
-
Constructed to withstand the challenges of motorhome life—vibration, temperature swings, and exposure to DEF’s chemistry
-
Designed for field replacement, minimizing downtime and restoring your emissions system quickly
While it may seem like just another sensor, the DEF head unit is central to the health of your aftertreatment circuit. Installing the correct OEM 4310-18-39 Tiffin Cummins X12 DEF Head Unit restores proper feedback, stabilizes SCR operation, and helps you travel with confidence—knowing your emissions system is doing its job.



















































































































































































































































