A clogged or restricted catalytic converter significantly increases exhaust backpressure. This increased backpressure forces the engine to work harder, generating more heat. This extra heat can stress the cooling system, pushing it to its limits. If the cooling system is already marginal (perhaps due to age, low coolant, or a slightly undersized radiator for the engine's demands), the added heat from the converter's restriction could cause overheating.
Since you've already replaced the radiator and thermostat (major components of the cooling system), the problem is less likely to be directly within the cooling system itself. However, the increased heat load from a clogged catalytic converter could still overwhelm even a newly installed radiator and thermostat.
In short: A bad catalytic converter is a *possible* contributing factor to overheating, especially if combined with other factors. It's not the most common cause, but it's definitely something to investigate if your Malibu is running hot despite having a new radiator and thermostat. Have a mechanic check the exhaust system's backpressure to rule out a clogged catalytic converter.
