The invention relates mainly to a method for monitoring a combined measuring probe (11) for an exhaust gas depollution system emitted by a heat engine (2) fitted to a motor vehicle comprising:a system (6) for reducing nitrogen oxides contained in the exhaust gases by reaction between ammonia and said nitrogen oxides, anda catalyst (10) for reducing an excess of ammonia contained in the exhaust gas, characterized in that said method comprises a step of selective switching between a first piece of information from the probe (11) relating to a measurement nitrogen oxides, such as the rate of nitrogen oxides, contained in the exhaust gas and a second piece of information from the probe (11) relating to a measurement of ammonia, such as the rate of ammonia, contained in the exhaust gas, depending on the operating conditions of the engine (2) and / or the motor vehicle.
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