Responding to legislation to reduce NOX emissions from diesel exhausts, some cars are fitted with NOX traps between the exhaust manifold and the DPF. These are maintenance-free and have no moving parts, but the downside is increased CO2 emissions and reduced engine efficiency. An alternative is selective catalytic reduction (SCR), which incorporates an electronic injector (as shown) that doses a distilled water/urea solution (Adblue) over a special catalyst material within the exhaust system. This chemical reaction takes place, once the exhaust gas temperature reaches around 170°C, or very soon after a cold engine start. By law, should the Adblue level be exhausted, or the SCR system malfunction, the engine will stop running, although the driver will receive plenty of dashboard warnings beforehand. From a DIY perspective, you may have to renew the injector if it develops a fault. Should you have to replace a DPF that incorporates SCR, pay attention particularly to the quality of the catalysts, because the car will not run, should a sub-standard exhaust be fitted.