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The code here indicates the ECM is faulty needing replaced.This is a straight forward code indicating an internal fault in the ECM itself.It may be when the vehicle is warm the ECM circuits internally act up/fail causing this here needing replaced.
Since this occured after a power wash I would check and make sure the ECM wiring connections did not get any water in them and that the wiring is all o.k.
The only other common fault that triggers this code is incorrect coding/programming for the ECM with a dealer needing to re code/reprogram it to correct this here but since this is intermittent this can be water in the connections needing dried out or a faulty ECM.
Water may have made its way into the ECM causing it to fail/act up like this here.
P1640 ECM faulty
NOTE: These faults concern faults in the Engine Control Module (ECM). If such faults occur replace the ECM.
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