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Low transmission oil can cause engagement issues as well as stalling when stopping from the converter clutch not getting enough fluid. You mentioned a leak, so it may be best to address that prior to diagnosing the stall and engagement concern. You mentioned lean codes, were those old codes or current codes? If its a vacuum leak causing the lean issue, you can find that with short bursts of flammable brake cleaner sprayed around all seals and gaskets and hoses. If you find a leak the rpms will change.
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