P1811 is a diagnostic trouble code (DTC) associated with the engine control module (ECM) on the Buick Lacrosse. This code indicates that there is an issue in the Engine Control Module's ability to communicate with various components of the vehicle, such as sensors and actuators. The exact cause of this code can vary depending on the make, model and year of your vehicle, so it is important to have a qualified technician diagnose and repair any issues related to P1811. Common causes for this code include faulty wiring or connectors, defective ECMs or sensors, worn out spark plugs or fuel filters, clogged catalytic converters, misfiring cylinders or vacuum leaks. In some cases resetting all codes may solve the problem but if not then further diagnostics will be needed in order to identify and rectify any underlying issues causing this DTC to appear.