A vehicle has set DTC P0172, indicating a lean exhaust. What is suspected for diagnosis?

The code P0172 indicates that the engine control module (ECM) has detected a rich condition in the exhaust, meaning there is excess fuel relative to the amount of air. When diagnosing this issue, one common suspect is a vacuum leak. A vacuum leak can cause the air-fuel mixture to become skewed; too much air entering the engine without sufficient fuel can lead to a lean condition, prompting the ECM to adjust by adding fuel.

When the ECM detects that it cannot maintain the ideal air-fuel ratio due to unmeasured air entering the system, it may signal that there's a problem in the intake system. This can lead to a series of compensatory actions by the fuel system, wrongly increasing fuel delivery to try to balance the mixture.

While other issues like fuel pressure problems or exhaust leaks could also lead to incorrect air-fuel ratios, a vacuum leak is more commonly associated with a P0172 error, since it typically results from unmeasured air diluting the expected fuel mixture. Thus, identifying and resolving vacuum leaks is a key step in addressing a P0172 code effectively.