Meaning of the engine P0156 Code is 02 Sensor Circuit Malfunction. The problem of the engine can be cause of several reason and those are Gas leaks in the finish, Air leaks in the intake, Defective fuel injectors, Broken or exposed wires, Voltage wires or circuits that have shorted out, Exhaust holes that are located near or around the oxygen sensor, Engine vacuum leaks and Oxygen sensor chains moving the exhaust. Verifying the car engine helps to identify the real problem of the car engine. Do not follow any wrong way for fixing the car engine. It is better if you take the car to professional automobile repair center.