B1878 means the restraints control module found voltage on the driver seat belt pretensioner circuit that has no business being there. Ford splits this circuit into three separate codes, and this is the short to battery version.
The module keeps the driver pretensioner circuit at a controlled state and watches it. B1877 is stored when the circuit reads open, B1879 when it finds a short to ground, and B1878 when it finds a short to battery or ignition voltage. Knowing which of the three you have narrows the search before you pull a single panel, because a short to power means something is feeding the circuit rather than something being disconnected.
Under the front seats is where most of these start. Power wiring runs through that area on a lot of vehicles, seat tracks move a few hundred times a year, and the carpet hides everything. A restraint wire that has lost its insulation against a seat frame edge and is touching a powered conductor produces exactly this code.
Circuit fault. Unlike an open pretensioner circuit, which frequently traces back to a deployment, a short to battery almost always points at a wiring problem you can put your hands on. The module took a live reading, so the fault is present right now.
No. A short to power is a live condition, and the module re-reports it every time it wakes up. Find the contact point, repair the harness properly with the right connectors instead of a twist and tape splice, and the code goes away on its own. If the same vehicle went through an accident and the module locked with stored crash data, that part is what our airbag module reset handles once the wiring is right.
This is a Ford-family code covering Ford, Lincoln and Mercury vehicles. Other manufacturers assign B1878 to a passenger seat cushion squib circuit shorted to positive, which is a different circuit in a different part of the car, so confirm the make before acting on a generic definition.
Move the seat all the way forward and all the way back with the code monitored live. A fault that appears at one end of the track has told you where to look. Pull the seat and follow the harness along the frame rails and under the carpet edge, and pay close attention to anything an installer added down there. Sort out the short before touching the pretensioner itself, since a good part will keep setting this code on bad wiring.
Related codes: B1877 (driver pretensioner circuit open), B1881 (passenger pretensioner open), and B0081 (center load limiter deployment control).
Related codes: B1877, B1881, B0081
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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