B1877 means the restraints control module checked the driver seat belt pretensioner circuit and did not get a valid reading back. On a car that has been in a collision, the fired pretensioner itself is usually what the module is seeing.
A pretensioner is a small pyrotechnic device that pulls the slack out of the belt at the beginning of a crash, before the airbag ever gets involved. Ford has mounted them in the buckle and in the retractor depending on the vehicle. Like an airbag inflator, it has an ignitor the module measures, and B1877 is stored when the driver side circuit reads open or otherwise outside the expected range.
Once a pretensioner fires, its ignitor is consumed and the circuit reads open forever after. That is the single most common source of this code on a vehicle bought after an accident, and it is a clue about the belt, not a wiring mystery to chase.
Ford also published a service bulletin covering 2001 to 2004 Escape models where the connection at the safety belt buckle pretensioner made marginal contact and set B1877 or B1881 intermittently, with replacement harness connector kits as the fix. Owners of that generation often noticed the light coming and going with the weather, since heat, cold and humidity move connector pins just enough to break a marginal contact.
Circuit fault, with a crash sitting behind it more often than not. The module is reporting a measurement it takes every key cycle. If a collision fired the pretensioner, that open circuit is the aftermath of the event rather than the record of it, and the record itself will be in separate crash codes in the same module.
Not on its own, and this is the code where the two halves of the job show up most clearly. The belt has to be made whole first. Our seat belt repair service rebuilds a fired pretensioner and returns the belt working like it did from the factory, and we also list pretensioner assemblies when a replacement is the better route. With the belt handled, our airbag module reset clears the locked crash data the same accident wrote into the module.
Ford, Lincoln and Mercury use B1877 for the driver seat belt pretensioner circuit, with B1878 and B1879 covering shorts on the same circuit. Some other manufacturers use B1877 for a front seat cushion squib circuit, which is a different part of the car entirely, so match the code to the make.
Look at the belt before the wiring. A pretensioner that has fired often shows a locked or shortened belt, a discolored buckle housing, or a retractor that will not pay out. If the belt is clearly fine, get under the seat and check the connector, and on an early Escape check whether the harness connector kit from Ford's bulletin was ever installed. Do not probe a pretensioner connector with a meter.
Related codes: B1878 (driver pretensioner short to battery), B1881 (passenger pretensioner open), and B0070 (driver pretensioner deployment control).
Related codes: B1878, B1881, B0070
Keep reading:
Code scope: Ford, Lincoln, Mercury. The same number can mean something different on another manufacturer, so match the code to your make before you act on it.
A reset clears stored crash data. A replacement part fixes a hardware fault. We have read these codes since the nineties and will tell you which one you are looking at.
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