OBD-II code family
Lost Communication With Restraints System Sensor (U0170–U0179)
10 codes describing the same fault on 10 enumerated slots. The letter is the index your manufacturer assigned — it is not a physical difference, and it does not carry across makes.
What these codes mean
Modern restraint systems do not rely on a single crash sensor. They use a network of them — front impact sensors in the bumper beam and radiator support, side impact sensors in the door pillars and sills, and on larger vehicles further sensors covering second and third row seating positions. Each one reports to the airbag control module over a dedicated network, and the module checks continuously that every sensor it expects to hear from is still talking.
This family of codes sets when one of those sensors goes silent. The letter in the code — A through J — is the slot number the manufacturer assigned that sensor, nothing more. It carries no generic meaning: Sensor D on a Ford is not Sensor D on a Toyota, and there is no industry table that maps letters to physical positions. The only authoritative source for which sensor your code refers to is the restraint system wiring diagram for your exact year, make, model and body style.
What the letter does tell you is something about your vehicle. A car that can report Sensor A or B is almost any car on the road. A vehicle that can report Sensor H, I or J has at least eight to ten restraint sensors, which in practice means a large three-row SUV, a luxury sedan with extensive curtain and side coverage, or a van. If your compact car is reporting a high letter, the code has most likely been misread, or your manufacturer has reassigned that number to something else — check it against your make before spending anything.
The diagnosis is the same regardless of letter, and it is more often wiring than sensor. Impact sensors live at the outer edges of the vehicle where water, road salt, and minor collision damage all reach them. A connector that has been soaked or a harness pinched during a bumper repair accounts for a large share of these codes. Because this is a supplemental restraint system, disable it and wait the manufacturer's specified time before disconnecting anything: these circuits fire pyrotechnic devices, and an accidental deployment causes serious injury.
Treat any of these codes as urgent. With one sensor missing, the module cannot make a correct deployment decision, and the usual response is to disable part or all of the restraint system — meaning airbags that may not fire in a crash. The warning light is not a suggestion.
Which restraints system sensor is mine?
There is no generic answer, and any source that gives you one is guessing. The letter is assigned by the vehicle manufacturer, and the same letter refers to different physical components on different makes — sometimes on different model years of the same vehicle. The only authoritative source is the wiring diagram or service information for your exact year, make, model and body style.
A manufacturer-specific definition always outranks the generic one. If your make publishes its own meaning for this code, use that and ignore the generic description.