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Things that can make the ECM *think* the A/F mixture should be RICHER than normal:
1) Engine HOT, ie: high coolant temperature (sensor bad).
2) High engine LOAD, ie: high TORQUE loading (engine rpm does not match throttle position).
3) MAP sensor is dirty.
4) Front Cat-sensor bad, ie: ECM thinks cat isn't up to operating temperature yet.
5) Back Cat-sensor bad.
6) One (or more) fuel injector(s) partly stuck open.
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