The B1901 trouble code in Toyota vehicles indicates an issue with the pretensioner circuit. This often points to a problem with the wiring or the pretensioner itself. This guide outlines how to test the resistance of the pretensioner circuit to help diagnose the B1901 code, specifically focusing on the driver and passenger side pretensioners. This test should be performed with the ignition OFF and the negative battery terminal disconnected for safety.
Important Note: The SRS ECU connector has a built-in safety mechanism that shorts the pretensioner circuit when disconnected. Therefore, keep the SRS ECU connector plugged in and disconnect the pretensioner connectors for this test. You will be testing the wires on the SRS ECU side of the connector. The image above shows the location of the connector for the pretensioners. The right side of the connector goes to the left pretensioner, and the left side goes to the right pretensioner.
Testing Procedure
Each pretensioner has two wires: a yellow wire and a yellow/black wire. You’ll need a multimeter to perform these tests. If your multimeter probes are too large for the connector, use a small piece of wire to extend the connection.
1. Continuity Test:
First, test the continuity of each wire. Connect your multimeter to each end of the yellow wire and then repeat for the yellow/black wire. A reading near zero ohms indicates a good wire.
2. Resistance Between Wires:
Next, measure the resistance between the yellow and yellow/black wires on the same connector. You should see a high resistance reading, typically in the megaohm range.
3. Resistance to Ground:
Finally, test the resistance between each wire (yellow and yellow/black) and a known good ground. This reading should also be relatively high, usually in the kilohm range.
Interpreting Results:
Readings outside the expected ranges suggest a potential problem within the pretensioner circuit, such as a short or open circuit. If you find any discrepancies, consult a Toyota repair manual for further diagnostic steps and repair procedures. Repeat this process for both the left and right pretensioner circuits. Remember to always prioritize safety when working with SRS systems. If you are unsure about any part of this process, consult a qualified automotive technician.
