Frank the tester they were using is a lambda tester. It gives the exact reading of lambda as processed by the control unit. A reading of 10 is indicative of a very rich mixture unless it is stuck there in which case it could very well be a faulty O2 sensor. This very well could be the problem in that if the computer thinks it is running too rich it could lean the engine out until it stalls...Thanks for getting back with us.
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