SPN 3512 — Sensor Supply Voltage 4

How to check it yourself

Sensor supply line No. 4 — stabilised 5 V that the ECU supplies to a group of sensors. If the line sags, all sensors on it drop at once, and there are often many codes.

What it should read

Check order

1. Write down all the codes: which sensors dropped together with line No. 4. They are its group — exactly which depends on the make and the engine.
2. FMI 4, 6 (short, voltage low): unplug the sensors of the group one at a time with the ignition on. When the voltage returns to 5 V after a sensor — that sensor or its wire is shorting the line.
3. FMI 3 (voltage high): the 5 V wire touches the vehicle's battery plus — look for a chafed harness where the sensor wires run alongside the power cables.
4. All sensors unplugged and the line still faulty — the wire from the ECU to the first junction, or the ECU itself.

Common causes — from most to least frequent

1. Shorted sensor on the line — most often the one in water or oil.
2. Chafed harness.
3. Water in the connector.
4. The ECU — rarely.