Emissions Reliability for Thomas Bus — OEM DEF Head Unit (04‑30774‑004), 2014–2024
What is it?
The OEM 04‑30774‑004 DEF Head Unit is a factory‑made header assembly for Thomas Built Buses equipped with Detroit DD5 or DD8 engines. This unit mounts to the DEF tank and houses sensors for fluid level, temperature, and other functions that support the SCR (Selective Catalytic Reduction) system’s emissions control.
Why choose this OEM DEF Head Unit?
Engineered to match Detroit and Thomas factory standards, this head unit ensures precise sensor behavior, correct sealing, and full compatibility with the vehicle’s DEF and diagnostic systems. Using the genuine component helps avoid issues like sensor faults, false DEF readings, fluid leaks, or SCR system warnings that often occur when aftermarket alternatives are used.
Key Benefits
✅ Direct‑fit for Thomas buses using Detroit DD5 / DD8 (2014–2024)
✅ Accurate monitoring of DEF fluid level and temperature
✅ Sealed construction to prevent leaks and contamination
✅ Compatible with factory diagnostics and emission control logic
✅ Durable build designed for vehicle vibration, thermal stress, and long service life
Applications
This head unit is ideal for Thomas school, shuttle, or transit buses powered by Detroit DD5 or DD8 engines that use DEF/SCR systems. It ensures dependable emissions system function across daily duty cycles, route service, and in fleet operations.
Installation Notes
A qualified technician should perform the installation. Begin by depressurizing or isolating the DEF system. Clean the DEF tank opening and inspect the mounting surface. Replace any worn gaskets or O‑rings. Mount the new head unit and torque the bolts to OEM specifications. Following installation, verify sensor readings via diagnostic tools, check for leaks, and clear any DEF or SCR fault codes to confirm proper operation.
Final Thoughts
If your Thomas bus is triggering DEF warnings, erratic level sensor readings, or DEF system faults, replacing the head unit with the OEM 04‑30774‑004 restores accurate emissions monitoring and system stability. With genuine construction and factory compatibility, it helps your bus stay compliant, reliable, and road-ready.