When diagnosing a vehicle with an STFT of +18% and an LTFT of +30%, what should be checked first?

In the context of diagnosing a vehicle with short-term fuel trim (STFT) at +18% and long-term fuel trim (LTFT) at +30%, the first step should be to check the fuel pressure. This choice is grounded in the understanding of fuel trim readings and their implications for fuel system performance.

When the STFT values are positive, it indicates that the engine control module (ECM) is adjusting the fuel delivery upward to compensate for a lean condition, while the substantial long-term trim suggests that this adjustment has been needed consistently over time. A lean condition often results from insufficient fuel delivery, which could stem from low fuel pressure. By checking the fuel pressure first, you can determine if the fuel pump is functioning properly and if the fuel filter is not clogged, both of which are critical to ensuring the engine receives the correct amount of fuel.

If the fuel pressure is found to be inadequate, it could directly explain the elevated fuel trim readings. If fuel pressure checks out, other components like vacuum leaks or the mass airflow sensor could be addressed next, but starting with fuel pressure is crucial given its direct impact on fuel delivery.