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VW Jetta Hesitates at Startup When Cold after Catalytic Replacement

I brought my 2005 MkIV Jetta in to the shop after living through a winter of the passenger compartment smelling like exhaust fumes. The mechanic said that the hose connecting my catalytic converter to the engine had cracked and needed to be replaced. Unfortunately, the hose is part of the entire catalytic converter unit so it had to be replaced. While replacing the cat, the mechanic also had to rip out one of the O2 sensors because it got stuck.

After this expensive repair, my car hesitates every time I attempt to start it while cold. As it’s been getting warmer at night this issue has mostly gone away now, but when it got down to 20-30 at night the engine did not sound good at all. It turns over just fine, but the first few seconds of running sounded like it was starved of one part of the combustion equation.

When I told my mechanic he said “replacing the cat shouldn’t have made it do that, let us have it overnight so we can start it cold in the morning one day and try to diagnose it.” I haven’t had a chance to do that, and now that it’s warm I probably won’t until fall rolls around.

What do you suppose could be causing this problem and is there a simple fix for it that I can try? Or should I wait until colder temperatures kick in and give it to the mechanic, hoping he’ll fix the problem?