P1659

P1659 is a Ford-specific code meaning the PCM detected a fault in the Cooling Fan 4 control circuit — specifically the high-side driver (HSD) inside the PCM or the FCM (fan control module) failing to source the commanded current to the fan relay or PWM-driven brushless fan. On SuperDuty Powerstroke trucks this is the auxiliary/secondary fan stage that ramps in under high IAT or trans-temp load. Shop-floor ladder cheapest-first: (1) Visual + connector check — pull the FCM/fan-relay harness connector on the LH front of the radiator support, look for green corrosion or melted pins ($0, 10 min, catches roughly 35% of P1659s on 2003+ 6.0L/6.4L trucks where road salt eats the connector). (2) Key-on engine-off, command the fan on with a bidirectional scan tool and watch fan current on live data — a healthy clutched/electric Ford fan pulls 5-10A at low speed and 18-25A at full command; under 3A means open circuit (broken HSD or harness break), above 30A means a shorted motor or seized bearing dragging the driver into thermal shutdown. (3) Back-probe the FCM control wire — should see 0V key-off, then a PWM duty cycle from the PCM rising from ~10% to ~95% as commanded temp climbs. No PWM = PCM driver dead. Steady 0V or steady 12V with no modulation = harness short. Expensive-misdiagnosis caveat: before condemning the PCM for an internal HSD failure on a 6.0L Powerstroke, pull the FICM and FCM connectors and check for the well-known 'pin-fretting' pattern — Ford's HSD diagnostic logic will set P1659 when the FCM itself has a stuck relay or shorted MOSFET, and shops have replaced $1,800 PCMs only to find a $180 fan control module was the actual fault.

Vehicle-specific patterns: 1999-2007 Ford F250/F350/F450 SuperDuty with the 7.3L and 6.0L Powerstroke uses a dual cooling stack (engine-driven clutched fan + electric pusher/auxiliary), and P1659 specifically targets the electric stage — failure modes are corroded FCM connector on the LH radiator support, water intrusion into the FCM itself, and PCM HSD failure after a jump-start with reversed polarity. 2008-2010 F250/F350 6.4L Powerstroke shares the same FCM architecture but adds a viscous-clutch sensor circuit that throws P1659 alongside P1247/P1248 when the clutch hub bearing fails. 2008-2010 Ford Excursion (final-year 6.0L diesel chassis) sees P1659 from the same FCM corrosion path as the SuperDuty. 2011-2019 F250/F350 6.7L Powerstroke moved to a brushless PWM fan with integrated controller — P1659 on these trucks almost always means the fan-internal driver board has failed and the whole fan assembly is replaced as a unit (no serviceable controller). Estimated repair: $180 to $2,400.