One problem related to software has been reported for the 2009 Mercury Mariner. The most recently reported issues are listed below. Please also check out the statistics and reliability analysis of the 2009 Mercury Mariner based on all problems reported for the 2009 Mariner.
Vehicle suddenly loses power steering assistance while driving. If the ignition key is cycled the power steering assist may function again. The affected component was determined to be the power steering control module (pscm) within the steering column. A recall was issued for this in 2014, however only the software was updated when the vehicle was brought in for repair. The malfunctioning components were not replaced and the issue has returned.