B1794: Occupant Classification ECU Power Line

Circuit fault

What B1794 means

The occupant classification ECU needs a clean battery positive supply to do its job. That supply runs from a fuse, through the harness, under the carpet, and into the connector at the seat. B1794 is set when the module sees no voltage arriving on that line, which tells it the circuit is broken somewhere along the way rather than that a sensor has failed.

An open on a power line is usually mechanical damage or a connection problem, not a component that quietly wore out. Fuses blow for a reason, connectors back out when a seat is yanked, and wires under a carpet get crushed by seat bolts and by cargo that shifted. The ECU may test perfectly once it finally gets voltage.

Common causes

• A blown fuse feeding the occupant classification ECU
• An open or chafed wire between the fuse box and the seat connector
• A seat connector that was unplugged and left loose after seat removal
• A terminal backed out of the connector body at the ECU end
• Corrosion at the connector from a water leak or a spilled drink at the passenger floor
• A crushed conductor where the harness passes under a seat mounting bolt

Crash data or hardware fault?

B1794 is a live circuit fault. The module tests for voltage on that feed every time it runs its self check, so the code reflects the wiring as it is right now and it clears once real power comes back. Stored crash data works on the opposite principle: written once after a deployment, held permanently, and untouchable with a scan tool.

Does our reset clear it?

There is nothing for a reset to do while the power feed is open. The module is reporting a true electrical condition, and the code returns immediately after any clear. Find the open and repair it. Once the vehicle side is right, a mail in reset is only needed if the module is also holding deployment data from a crash.

Which makes throw it

Toyota and Lexus both use B1794 on vehicles with occupant classification. The fuse that feeds the ECU and the routing of that circuit vary by model, so pull the wiring diagram for the specific chassis before probing.

What to do next

Check the fuse for that circuit first, since it takes a minute and a blown fuse also tells you something else is shorted. With the fuse good, back probe the seat connector for battery voltage with the key on, then work backward toward the fuse box to find where it disappears. Repair the open, reseat every terminal, and verify the code stays gone through several key cycles before you call it done. Related codes: B1790, B1800, 0X93090D.