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Re: 1999 blazer running bad when cold
O2 sensors are a possibility for causing a misfire on a warm engine, but not on a cold start. The sensors take about 2 to 3 minutes to warm up, during that time the ECM ignores the O2 sensors. When an O2 sensor gets old, (100,000 miles or more) it becomes "lazy" causing the ECM to increase injector pulse width which richens the fuel mixture = poor fuel mileage.
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