The trouble code "P0172 system too rich" means there is too little oxygen in the exhaust. This error code is triggered by an oxygen sensor in bank one, the side of the engine with cylinder one.
According to Repair Pal, a "rich mixture" represents an air to fuel ratio of below 14.7 parts air to 1 part fuel. Normally, the engine control module makes adjustments to the mixture by increasing or decreasing the fuel injection. When the error is triggered, it means the control module is attempting to maintain the mixture, but the ratio is still too low. The computer is no longer able to automatically adjust the mixture to the correct ratio. A "rich mixture" can smell like rotten eggs.
Various malfunctioning sensors, such as the mass airflow sensor, manifold absolute-pressure sensor, oxygen sensor, coolant sensor, air temperature sensor or throttle-position sensor can cause the error code. Other possible causes of the error code include engine mechanical problems, a faulty fuel-pressure regulator, ignition misfires or problems with the fuel line or fuel injectors.
According to OBD Codes, receiving error code P0172 may result in no drivability problems, or it may cause misfiring. Some possible solutions to resolving the error include inspecting vacuum hoses, cleaning the mass air flow sensor, inspecting fuel lines for cracks and damage, checking the fuel pressure or cleaning the fuel injectors.