What fuel trim readings would confirm a catalytic converter issue rather than a fuel system problem?

Normal short-term fuel trim (STFT) and long-term fuel trim (LTFT) values suggest that the engine management system is maintaining the correct air-fuel mixture, meaning that the fuel delivery system is functioning properly. When both STFT and LTFT values are within the normal range, it indicates that the car is not compensating for an excessive amount of unburned fuel or a lean condition typically associated with a fuel delivery issue.

In situations where the catalytic converter is experiencing problems, the exhaust gases may not be processing correctly, leading to poor performance and increased emissions without affecting fuel trim readings significantly. As a result, the catalytic converter issue can manifest without showing abnormal fuel trim readings.

In contrast, high STFT and LTFT values would suggest that there is an excessive amount of fuel being delivered or that the engine is running lean, both of which point to a fuel system problem. Negative STFT readings alone do not specifically provide enough evidence for a catalytic converter issue because they could also imply a rich condition that the fuel system is trying to correct. Similarly, an increasing LTFT reading could indicate that the engine is trying to compensate for a persistent lean or rich condition, which would not necessarily confirm a catalytic converter fault.

Therefore,