Process for producing a fluid for the decontamination of heat engines, in particular diesel engines, which makes it possible to simultaneously carry out selective catalytic reduction of the nitrogen oxides present in the exhaust gases and also assistance in the regeneration of the particle filter by catalytic combustion of the soot particles deposited in the particle filter ("assistance in the regeneration of the PF" function), said fluid being provided in the form of a stable suspension comprising colloidal particles of one or more metal oxides or oxide/hydroxides or of metal carbonates dispersed in a solution of urea at 32.5% by weight in pure water, the metals of the metal oxides or oxide/hydroxide or carbonates being chosen from the list of following metals: Fe, Cu, Ni, Co, Zn, Mn, Ti, V, Sr, Pt, Ce, Ca, Li, Na and Nb, and preferentially from the following list: Fe, Cu, Ce and Sr, said process consisting of the sequence of following steps: • a) synthesis of the particles of metal oxides or oxide/hydroxides or carbonates, • b) functionalization of the particles of metal oxides or oxide/hydroxides or carbonates with organic molecules or by coating in order to stabilise these particles in the aqueous solution of urea, • c) purification by filtration, or filtration/washing, or rinsing, or dialysis, which processes are applied alone or in combination, of the suspensions of particles of metal oxides or oxide/hydroxides or carbonates, • d) dispersion of the functionalized particles of metal oxides or oxide/hydroxides or carbonates in said aqueous solution of urea, according to one of the methods below: ∘ mechanical stirring using a stirrer equipped either with a propeller shaft comprising a certain number of blades or with any other item of equipment known to provide efficient stirring of the suspension, ∘ stirring with a high-shear homogenizer-disperser, ∘ stirring with a colloid mill, • e) dispersion using an ultrasound probe, step e) being either carried out as a function of the result and of the effectiveness which are obtained on conclusion of the step of mechanical dispersion of step d) (in this case, this step e) is optional), or carried out in place of the step of mechanical stirring of step d). • a) synthesis of the particles of metal oxides or oxide/hydroxides or carbonates, • b) functionalization of the particles of metal oxides or oxide/hydroxides or carbonates with organic molecules or by coating in order to stabilise these particles in the aqueous solution of urea, • c) purification by filtration, or filtration/washing, or rinsing, or dialysis, which processes are applied alone or in combination, of the suspensions of particles of metal oxides or oxide/hydroxides or carbonates, • d) dispersion of the functionalized particles of metal oxides or oxide/hydroxides or carbonates in said aqueous solution of urea, according to one of the methods below: ∘ mechanical stirring using a stirrer equipped either with a propeller shaft comprising a certain number of blades or with any other item of equipment known to provide efficient stirring of the suspension, ∘ stirring with a high-shear homogenizer-disperser, ∘ stirring with a colloid mill, ∘ mechanical stirring using a stirrer equipped either with a propeller shaft comprising a certain number of blades or with any other item of equipment known to provide efficient stirring of the suspension, ∘ stirring with a high-shear homogenizer-disperser, ∘ stirring with a colloid mill, • e) dispersion using an ultrasound probe, step e) being either carried out as a function of the result and of the effectiveness which are obtained on conclusion of the step of mechanical dispersion of step d) (in this case, this step e) is optional), or carried out in place of the step of mechanical stirring of step d).