Under which condition is EGR typically disabled?

• A. Excessively high engine coolant temperatures
• B. Low engine coolant temperatures
• C. Extreme engine oil viscosity
• D. Low intake air temperature

Answer: (A)

Exhaust Gas Recirculation is used to lower NOx by recirculating a portion of exhaust gas into the intake, which reduces peak combustion temperatures. But the engine control system will disable EGR when the engine is overheating. If the coolant temperature is excessively high, continuing to recirculate exhaust gas can add unwanted heat to the intake charge and complicate cooling, potentially worsening overheating or causing unstable combustion. Turning EGR off in this condition helps protect the engine and keep operation stable while it cools down.

The other factors—very cold coolant, extreme oil viscosity, or very low intake air temperature—don’t trigger EGR disable in this context, so the high coolant temperature condition is the one that best explains why EGR is typically turned off.