Emissions from modern SI-engines are reduced by a three way catalyst. However if there are leaks in the exhaust system before the catalyst emissions increase for two reasons. First the untreated emissions leak out. Second which is worse, due to waves in the exhaust system, oxygen leaks into the manifold and causes an oxygen sensor offset. The result is increased emissions as the air/fuel controller makes the engine run rich. Here a method to detect leakages in the exhaust manifold is presented. The sensors used are binary oxygen sensor(s), intake manifold pressure and temperature, and the air mass flow sensor. Injection time is also used to estimate air/fuel ratio. Experimental results are shown with measurements from a turbo charged SAAB SI production engine with wastegate.