The Duratec 2.0 HE aka Mazda L-Series engine is designed to feature an EGR (Exhaust Gas Recirculation) valve to recirculate the engine’s exhaust gas partly back to the intake port reducing emissions. The valve is electronically controlled by the ECU and is cooled by the engine’s cooling water.
Caterham does not use an EGR valve in the CTI (Caterham Technology and Innovation) spec. Duratec 2.0 175 engine. The according port in the block is closed by a blanking plate. This increases power but also increases emissions slightly.