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A friend's catalytic converter was stolen, her mechanic is about 35 miles away. What are the risks of driving the car to the mechanic (instead of having it towed)?
I'm assuming that it would be unpleasantly loud, would there also be a realistic risk of carbon monoxide poisoning? Any other risks?
I don’t think I would drive it any distance. You are going to have very hot gasses exiting close to the under body sealant (underseal). This is likely to catch fire and spread quickly.
You can bet that with the converter removed, it's going to be unpleasantly loud; so loud that the operator is likely to be as light on the go-pedal as ever she was.
The greatest risk for such a circumstance, with respect to CO danger is enclosed areas and stagnation while not moving.
For normal driving, with the average number of traffic lights, the level of stagnation is quite low. Traffic in the area will stir up a wind, ambient weather will often be not-calm. There's not much snow with which to enclose the vehicle, preventing another type of closed area.
The risk is extremely low, therefore, conceivably non-existent.
With a poorly tuned engine, there could be a risk of fire, although converter fires are much greater than spark-ignited fires from a chopped-open exhaust system. Don't park on dry grass with the engine running and zero out that risk as well.
As Handy Howard mentioned, you will have exhaust gases exiting where the thief cut off the catalytic converter. This may damage components or wiring around that area.
There is also a risk of the exhaust gases entering the passenger compartment and causing carbon monoxide poisoning. The driver could pass out while driving and crash.
Yet another possibility is getting pulled over for illegal or modified exhaust. I'd hope an officer would be understanding in this case, but you never know.
You are fine to drive to your mechanic. The only issue is your engine will not perform optimally because your O2 sensors will not operate within the intended range. This will trigger your check engine light to go on. Your mechanic will be able to reset your check engine light so it goes off.
