On Board Diagnostics: Outputs

When at rest, when a last-stage transistor is not activated, the voltage between collector and emitter must be high (often battery voltage). When the transistor is active, the voltage must be low. Otherwise a diagnostic trouble code is set.

Here, the control module first has to have learned that the car is fitted with A/C. This is saved to the RAM. Cars without A/C must not have this diagnostic trouble code stored, although they do not have an A/C relay.

For a diagnostic trouble code regarding interference to be set, the output must be deactivated. The correct voltage for an active output is 0 V.

If the voltage is too high in the case of an active output, a diagnostic trouble code for a short-circuit to B+ is set. This could be due to a burnt relay winding, a short-circuit in the network or made an incorrect connection in the relay base during troubleshooting.

"Warm-up" is short for the Warm-Up Cycle and is described in US legal requirements. A warm-up involves raising the engine temperature by 20°C, and the temperature also has to be in excess of 71°C. If the fault is rectified without deleting the diagnostic trouble code, the diagnostic trouble code will remain in the control module fault memory for 40 warm-ups.