Fault code P0172
There are many potential problems that can cause this error code to be generated. The optimum air-fuel mixture is determined by a pre-conceived 'map' that the engine management computer refers to about 10 times per second. It then compares sensor information to the reference values in the map and adjusts the fuel-air mixture accordingly. Therefore, any single factor that can influence the reference information the computer receives from sensors could potentially be the problem. Now, it's a process of elimination / testing. First, start with the vacuum system. Ensure there are no vacuum leaks. Next, make sure all electrical connections and wiring are in good condition AND, especially for volkswagen, make sure the grounding points for the electrical system are all clear of corrosion and grounding nicely. Finally, if the problem is still there, refer to a service manual for your vehicle and use a multi-meter to check sensors such as the mass airflow sensor or manifold absolute pressure sensor, air temperature sensor, throttle position sensor, engine temperature sensor, crankshaft and camshaft position sensor, oxygen sensors, etcetera, for errors. In most cases, if there is a fault in any one of these sensors, it will register an error code, BUT NOT ALWAYS!! If there are no vacuum leaks, the wiring is good, and all sensors spec as functioning appropriately, then you may have a problem with your ECU (although I doubt it's the latter). Good luck, now get to hunting for your problem!
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