A vacuum leak can cause an O2 sensor to set a fault code, which may trigger the check engine light to come on. The O2 sensor monitors the oxygen concentration of the exhaust gas and sends a signal to the engine management system to indicate the air fuel blend. If the air fuel mixture is not correct due to a vacuum leak, The engine control module (ECM) may use the lambda sensor to monitor the operation and set a code if necessary. The O2 sensor may set a code to alert the driver or mechanic of the problem if the issue is not resolved.