What could indicate a rich mixture in a vehicle leading to poor catalyst function?

High fuel trim values indicate that the engine's fuel mixture is richer than intended. Fuel trim is the adjustment made by the engine control unit (ECU) to keep the air-fuel mixture within an optimal range for combustion. In a normally functioning system, trim values should be close to zero, suggesting that the engine is receiving the appropriate mixture of air and fuel.

When the fuel trim values are high, it suggests that the ECU is compensating for an excessive amount of fuel being delivered to the combustion chamber. This could be the result of various factors such as a leaking fuel injector, a malfunctioning MAF (Mass Air Flow) sensor, or other issues that affect the air-fuel balance. A rich mixture leads to incomplete combustion, causing unburned fuel to enter the exhaust system. This excess fuel can overheat and damage the catalytic converter, which impacts its ability to convert harmful exhaust emissions effectively.

By monitoring fuel trim values, technicians can assess the air-fuel mixture health and potentially identify underlying issues affecting catalyst performance. This makes understanding high fuel trim values crucial for diagnosing rich mixture conditions and addressing poor catalyst function.