What's the BMW fault code p13b4?

On a BMW, the diagnostic trouble code P13B4, is shown when the ECM senses that the Crankshaft / Inlet Camshaft Position Sensor is outside of factory specifications. This can happen for a number of reasons, the sensor may have gone bad, the harness to the sensor may have been damaged or the connection between the sensor and the harness may have corroded and lost electrical contact. The first thing You want to do is check the connection, disconnect the plug and make sure there is no water or debris inside. Also make sure nothing has physically damaged the sensor. If everything looks ok then Your neXTStep would be to replace the sensor. You can usually find a replacement online for a lot cheaper than buying directly from BMW.