In what case would a technician conclude that a catalytic converter needs to be replaced?

A technician would conclude that a catalytic converter needs to be replaced when the temperature difference between the inlet and outlet is very low. This temperature differential is an important indicator of the catalytic converter's efficiency in converting harmful exhaust gases into less harmful emissions.

Typically, a functioning catalytic converter will show a significant difference in temperature between its inlet and outlet—indicating that the catalyst is effectively facilitating chemical reactions that reduce pollutants. When the temperature difference is minimal, it often suggests that the catalyst is either damaged or contaminated and is no longer performing its intended function, which is to convert and reduce emissions.

High exhaust temperatures (as mentioned in the first option) can be a sign of other issues such as engine problems or running too lean or too rich, but do not directly indicate the failure of the catalytic converter. Optimal fuel consumption (the third option) may coincide with a properly functioning exhaust system, but it does not provide direct evidence of the catalytic converter's condition. Lastly, the absence of diagnostic trouble codes (the fourth option) doesn’t guarantee that the catalytic converter is performing well; some failures may not trigger a code, especially in cases where the efficiency drop is gradual or within tolerances that the vehicle's onboard diagnostics can't detect.