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Diago
Instant AI vehicle diagnostics
Diago
Instant AI vehicle diagnostics
A check engine light on a Class 7 or Class 8 truck is a different beast than a car. DEF faults, air brake warnings, DPF regeneration issues, and ECM codes all need to be read the right way — especially when you’re on the road and miles from a truck stop.
Low DEF level, bad DEF quality, failed DEF pump or injector, or SCR catalyst fault. Leads to a 5 mph derate if ignored.
Diesel Particulate Filter gets clogged and can’t complete a regen cycle. Causes power loss, high exhaust backpressure, and potential engine damage.
Extremely common on high-mileage diesels. EGR cooler failures can cause coolant loss and severe engine damage if not caught early.
Low boost, over-boost, or turbo lag codes. Often caused by a failing VGT actuator, boost leak, or clogged intercooler.
Injector return flow issues, HPCR (high pressure common rail) pressure faults, or fuel filter restriction codes are common on high-mileage trucks.
NOx sensors and other emissions sensors are wear items on modern diesels. Failed sensors often trigger derate sequences on newer trucks.
Diago supports all major Class 7–8 makes and engines:
Tell Diago your truck, engine, and what the dash is showing. Get an instant diagnosis with the most likely fault, what it means for drivability, and the nearest truck stop repair options.
Diagnose my semi truck →Check your dash for derate messages and warning lamp color. Yellow means monitor it; red means stop safely. A DEF fault with no derate yet gives you a window to reach a shop. A red engine lamp or active derate means the ECM is already limiting power to prevent damage.
Low DEF level, bad DEF quality (diluted or contaminated), failed DEF pump, or an SCR catalyst fault. A DEF fault left unaddressed on most modern trucks will trigger a 5 mph derate within a set number of miles or engine hours.
Yes — Diago’s big rig mode includes links to FMCSA compliance resources, roadside inspection categories, and CDL-relevant safety information alongside the technical diagnosis.
The resources panel in Diago shows the nearest Pilot / Flying J, Love’s, and TA/Petro truck stops with service bays, plus roadside assistance links — so you’re not just getting a diagnosis, you’re getting a next-step plan.