Method and system for detecting the absence of under engine protection

A system for diagnosing the absence of an under-engine protection of a motor vehicle, comprising:

• first measurement means for measuring or estimating a temperature indicative of the temperature of air external to the vehicle,
• second measurement means for measuring or estimating a temperature indicative of the temperature of the air passing through the engine compartment of the vehicle, referred to as the under-hood temperature,
• first comparison means for determining whether the external-air temperature value is below the under-hood temperature value,
• calculation means for calculating, if the output from the first comparison means is in the affirmative, a first absolute value of a difference between the external-air temperature measurement and the under-hood temperature measurement,
• determination means for determining a first diagnostic criterion by calculating a difference between said first absolute value and a predetermined first absolute value for a difference between the external-air temperature and the under-hood temperature, obtained in the presence of the under-engine protection,
• second comparison means for comparing the first diagnostic criterion against at least one first predetermined diagnostic threshold, and
• emission means for emitting a diagnostic signal dependent on the output of the second comparison means.