Diagnostic code P0172 indicates that bank one is too rich. An engine is said to run "rich" when the air-to-fuel ratio goes below the ideal 14.7 to 1 ratio with too much fuel in the mix. Gasoline engines are always trying to maintain a good ratio of fuel and air, and when the ratio of fuel goes beyond normal limits, a rich code will set and a service engine soon light will illuminate.
Most engines use oxygen sensors in the exhaust stream to determine if the system is running rich or lean at any given moment. The engine management system can make adjustments to the air fuel mixture based on data inputs it receives. Information such as engine temperature, as well as intake air volume and engine load can be used to determine if more or less fuel needs to be delivered to the engine.
A rich code means that too much fuel is being provided to the engine and a high percentage of incompletely burnt fuel is leaving the tailpipe.
Since an ignition source is needed for combustion in gasoline engines, a good first step in the diagnosis of rich codes would be a complete inspection of all parts related to ignition. Bad spark plugs, ignition coils, ignition wires, etc., could lead to a rich running condition.
Fuel system checks are also commonly performed when diagnosing code P0172. Leaking fuel injectors and ruptured fuel pressure regulator diaphragms are among the common culprits for this code. It's common to see some G.M. engines with leaking fuel supply lines and regulators leaking under intake plenums, resulting in codes P0172 and P0175, which indicates a rich condition on both banks. Fuel pumps that produce too much pressure, either from a blocked return line or a bad regulator can also produce rich codes.
Sensors can cause rich codes too. Sensors that are used to measure oxygen in the exhaust, intake airflow, manifold pressure (used to calculate air density), throttle position, engine temperature are all suspect when diagnosing rich codes. For example, if a temperature sensor was reporting to the computer that the engine was operating at -40 degrees Fahrenheit when it was actually operating at 200 degrees, this could cause the computer to "dump" more fuel than is needed. Likewise, a bad MAP sensor or vacuum supply problem to the sensor could convince the computer that the engine was operating at a higher altitude or under heavy acceleration.
Basic Mechanical Checks...
Since engine problems can also cause poor combustion and low vacuum, engine compression tests may be needed to uncover any internal engine problems which might lead to a rich running condition and code P0172.