The P0739 code in your 2021 Lincoln Navigator indicates low voltage in the transmission control module enable circuit. This is the opposite of P0738—instead of high voltage from an open circuit, low voltage suggests a short to ground or excessive current draw that pulls voltage down.
Low Voltage Circuit Conditions
When the enable circuit shows low voltage, current is being diverted to ground through an unintended path. This could be from damaged wiring touching metal, water intrusion creating ground paths, or internal circuit failures that short to ground. The result is inadequate voltage to properly enable the TCM.
Common Causes
A short to ground in the enable circuit wiring is the primary cause. Corroded connectors with moisture paths act as shorts. Damaged insulation allowing wire-to-chassis contact. The TCM's internal enable circuit may have failed in a shorted condition. Water intrusion into wiring or connectors creates unintended ground paths.
Symptoms in Your Navigator
Transmission may enter limp mode or failsafe operation. Shift timing and quality may be affected. Multiple warning lights may illuminate. In severe cases, the TCM may not operate at all. Symptoms may be intermittent if the short only occurs under certain conditions.
Ford Diagnostic Approach
Using Ford IDS/FDRS, check TCM power and enable circuit status. Measure voltage at the enable circuit—low readings confirm the condition. Disconnect circuit segments to isolate the short location. Check wiring for damage, especially where it routes near metal edges or water exposure areas. Verify TCM grounds are secure and corrosion-free.