If a vehicle shows an LTFT of -15% and fails a smog test, what might be the underlying issue?

When a vehicle displays a Long Term Fuel Trim (LTFT) of -15%, it indicates that the engine's control module is reducing the amount of fuel injected into the engine by 15% to compensate for a perceived rich condition. This means that the engine is likely running with more fuel than necessary, which could be a sign of an underlying issue.

A clogged catalytic converter aligns with this condition because it can cause increased backpressure in the exhaust system. If the catalytic converter is unable to effectively process the exhaust gases, it can lead to a buildup of gas in the engine, prompting the engine control unit (ECU) to reduce the fuel injection to maintain the correct air-fuel ratio. This results in the negative fuel trim reading, as the ECU attempts to adjust fuel delivery due to excessive unburned fuel backing up in the system.

The other choices do not support the observed LTFT reading of -15% effectively. Excessive fuel pressure, for instance, would typically cause a rich condition leading to a positive fuel trim adjustment instead. Leaking coolant is more related to issues affecting engine temperature and performance without directly influencing the fuel trim, while normal operating conditions would not produce such a negative fuel trim reading. Thus, a clogged catalytic converter is