An aftertreatment module is disclosed. The aftertreatment module may include a housing and a mounting plate within the housing that forms an inlet chamber and an outlet chamber. The aftertreatment module may include an inlet for exhaust gas from a combustion engine to flow into the inlet chamber and an outlet through a top plate of the housing for the exhaust gas to flow from the outlet chamber, wherein the outlet comprises a plurality of perforations. The aftertreatment module may include an outlet sensor mounted on the outlet to obtain information relating to the exhaust gas as the exhaust gas flows from the outlet chamber and a set of catalysts mounted to the mounting plate to treat the exhaust gas as the exhaust gas flows from the inlet chamber to the outlet chamber. The aftertreatment module may include a drain port through a side plate of the housing.