The engine is running rich on bank 1, meaning the air to fuel ratio is biased towards fuel.
What P0172 really means:
Over time when the engine wears, parts break, or if measuring sensors become faulty, the computer can make corrections by shortening the on time for the fuel injectors to lean up the mixture. This code can also be accompanied by a P0175, which will identify bank 2 as rich. The ideal stoichiometric ratio is 14.7 parts of air to 1 part of fuel, which allows a good mixture of fuel economy, performance and low emissions. The computer can make corrections for this condition up to a certain point by shrinking the on time for the fuel injectors.
What are the symptoms of a P0172 code?
What we will notice right away is a sulphur like smell and black exhaust. These are two of the most common symptoms, and are signs that unburnt fuel is overwhelming the catalytic converter. Unlike a lean code, where the problem could be either too little fuel or too much air, rich codes typically signify an issue with over-fueling: fuel injectors are not sealing properly and leaking fuel into the cylinder, the fuel pressure regulator is bad and leaking fuel into the intake or there is a bad sensor that fools the computer into believing there is more air coming into the engine then there really is.
What is the severity of a P0172 code?
Significant. The car should be looked at in a timely fashion, but unless the car is misfiring or the check engine light is blinking an appointment can be set up at your convenience.
What type of repair is required to resolve a P0172 code?
A diagnostic check with a qualified mechanic is required to pinpoint the exact issue. There is a decent sized list of reasons that an engine will run rich. There are some usual suspects fuel injectors, fuel pressure regulators plus mass air flow and oxygen sensors.
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_Service article written by an ASE Master Technician_