P0691

P0691 means the PCM detected the Fan 1 control circuit pulled low when it should not be, or the low-side driver is reporting under-voltage on the cooling fan 1 output. Cheapest-first ladder: key-on engine-off, unplug the fan 1 relay or fan control module connector and measure resistance across the relay coil (typically 50 to 90 ohms) or the FCM low-side driver pin to ground (should not be a dead short). With the connector unplugged, the PCM control wire to the relay/FCM should sit at battery voltage (about 12.6 V engine-off, 13.8 to 14.2 V running); 0 V at this point points to a wiring short to ground between PCM and relay, which is the most common P0691 cause. Reconnect and command the fan on with a scan tool; on a healthy single-speed setup you should see 0 V on the control wire when commanded on (PCM is pulling it to ground) and battery voltage when commanded off, with fan current draw between 8 and 25 A momentary inrush settling to 3 to 12 A steady, and less than 0.3 V drop across the fan power feed terminal under load. On PWM-controlled fans (Ford F150 5.0 Coyote, GM 6.2 L86) watch duty cycle on the scan tool ramp from 0 to 100% as commanded ECT rises from 200 to 235 F. Expensive-misdiagnosis caveat: fan resistor packs are the common consumable on multi-speed fan systems. Ford, GM, and Toyota all use 3-step fan resistor packs (low/med/high) that burn open at the high-current stage from heat-cycling at the soldered tab; the fan itself runs fine on low and medium, the PCM commands high, sees no current flow, and sets P0691 or P0693. Replace the pack ($40 to $120), not the whole fan assembly ($250 to $600).

Vehicle-specific patterns: 2011-2014 Ford F150 5.0 Coyote uses a PWM single-fan setup through the FCM-A relay/module mounted on the radiator support; the FCM-A solders to its own pigtail and the solder joints crack from underhood heat, setting P0691 intermittently when the connection opens; Ford supersession is a redesigned FCM with crimped terminals, $150 to $260 installed. 2007-2014 GM Silverado/Tahoe/Yukon with the 5.3/6.0 use a clutched primary fan plus an electric secondary fan; P0691 on these is almost always the electric secondary relay (mounted in the underhood fuse block) or its low-side driver shorted to ground after a coolant leak from the upper hose dripped onto the relay block, $35 relay swap. 2007-2017 Toyota Camry V6 (2GR-FE) runs a dual electric fan with a 3-step resistor pack riveted to the fan shroud; P0691 here is the pack 80% of the time, $50 to $90 part. 2006-2015 Honda Civic/Accord with the single PWM 1A or 2A fan throws P0691 from chafed fan-motor harness where it routes past the radiator support bracket, repair the chafe and the code clears without parts. Estimated repair: $35 to $480.