B1790: Occupant Classification ECU Communication

Circuit fault

What B1790 means

The occupant classification ECU decides whether the front passenger seat holds an adult, a child seat, or nothing, and it sends that decision to the center airbag sensor assembly. The center assembly uses the answer to choose how the passenger airbag behaves in a crash. B1790 means the center assembly is no longer receiving usable data on that communication line.

Because this is a communication path, the fault can sit at either end or anywhere between. A dead classification ECU stops transmitting. A damaged wire under the carpet interrupts the signal. A connector at the seat rail that was unplugged for seat work and never latched properly produces the same complaint. The module cannot tell you which of those it is, only that the conversation stopped.

Common causes

• The connector under the front passenger seat left unlatched or damaged after seat removal
• Wiring under the carpet crushed, pinched by a seat bolt, or cut during floor work
• An occupant classification ECU that has failed and no longer transmits
• Water intrusion at the passenger floor pan reaching the ECU or its connector
• Corroded or backed out pins at the center airbag sensor assembly connector
• A classification ECU installed from another vehicle and not matched to this one

Crash data or hardware fault?

Communication circuits get evaluated live, which separates this code from stored crash data. The module evaluates the link continuously, so the code follows the actual condition of the wiring and the two modules. Crash data, by contrast, is a one time entry that the module keeps forever after a deployment and that only a reset removes.

Does our reset clear it?

A reset does not restore communication between two modules. As long as the line is broken or one end is dead, B1790 comes right back when the system runs its next check. Repair the harness, the connector, or the failed ECU on the vehicle. Our mail in service is the right answer when the same module is also carrying crash data from a deployment.

Which makes throw it

Lexus and Toyota both list B1790, and it applies to vehicles equipped with the occupant classification system. The ECU is typically part of the front passenger seat assembly, so seat service on either make is a common way to introduce this fault.

What to do next

Slide the passenger seat forward and back and inspect the connector at the seat base, because an unlatched plug after seat work explains more of these than any failed part does. Follow the harness along the floor for pinch damage at the seat mounting points. Check for power and ground at the classification ECU before condemning it. If the ECU is confirmed dead, we can source a genuine OEM replacement and reset the airbag module if it is holding crash data as well. Related codes: B1787, B1794, B1382.