Cat monitor will not run, even though i've done the 55 mph for 5 minutes or more numerous times....
My car has 235000 miles.
My car has an automatic transmission.
If the cat monitor will not finish and pass after multiple tries then you may have a weak sensor or a catalyst that is bad. I'd suggest having the vehicle scanned to test the O2 sensors for slow or limited operation. If you decide to get this fixed, feel free to contact a certified mechanic who can diagnose these conditions for you and replace your catalytic converter if necessary. Good luck.
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