Exhaust backpressure detection

Disclosed is a method of controlling operation of an after-treatment apparatus for an internal combustion engine. The after-treatment apparatus comprises a Diesel Particulate Filter (DPF), a Selective Catalytic Reduction (SCR) module downstream of the DPF and an injector for injecting reductant upstream of the SCR module. The after-treatment apparatus also comprises a first pressure sensor upstream of the DPF and configured to output a first pressure value representative of pressure of gas entering the DPF and a second pressure sensor configured to output a pressure value representative of a pressure difference over the DPF. A controller is configured to calculate an inferred pressure value indicative of a pressure downstream of the DPF based on the pressure of the gas entering the DPF and the pressure difference across the DPF. The method comprises comparing the inferred value of pressure downstream of the DPF with a reference pressure value when a mass flow through the DPF exceeds a mass flow rate threshold value. The reference pressure is dependent upon the mass flow through the DPF. The difference between the inferred downstream pressure and the reference pressure and a selective catalytic reduction module regeneration or maintenance indicator is triggered based on the result. An after-treatment system and computer program for carrying out the method are also disclosed.