For operators of Thomas school buses powered by Cummins ISB 6.7L, ISL, or L9 engines, emission system reliability is a critical priority. The DEF (Diesel Exhaust Fluid) head unit is one of the unsung heroes in that system. It ensures the engine control unit (ECU) receives accurate data on fluid level, temperature, and urea concentration—without which your SCR (Selective Catalytic Reduction) system can’t function properly. A failing DEF head can trigger warning codes, force derates, or even lockouts during critical operations.
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Monitors DEF fluid properties (level, temperature, urea concentration) and sends real-time data to the ECU
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Regulates precise dosing of DEF into the exhaust stream to enable correct NOx reduction via the SCR system
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Built to match OEM tolerances for Thomas Bus / Cummins ISB, ISL, L9 platforms, ensuring plug-and-play compatibility
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Helps prevent fault codes, emissions warnings, or engine derates stemming from sensor drift or inaccurate data
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Designed to withstand vibration, thermal cycling, and exposure to DEF’s corrosive chemistry
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Allows for field replacement to minimize downtime and keep buses running reliably
In the world of transit and school bus service, every minute out of commission costs money and impacts safety. Equipping your fleet with properly functioning DEF head units ensures your emissions control remains precise, compliant, and dependable. Don’t let a degraded sensor compromise your system—make your DEF head unit a maintenance priority.


















































































































































































































































































