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.

To bear in mind in respect of On Board Diagnostics: 

• Not all faults that arise can be identified. Even if there is no electronic fault for an injector, it does not necessarily mean that it is injecting petrol as it is supposed to. It only indicates that the circuit is OK.
• In general, digital inputs have no or poor diagnostics. Use the diagnostic tool to check the read values.
• In general, analog inputs have good diagnostics. One knows that the circuit is OK if no diagnostic trouble code is set. One cannot, however, be sure that the value is correct. The fact that no diagnostic trouble code is set can be misleading. Use the diagnostic tool to check the read values.
• Outputs generally have good diagnostics. You can be fairly certain that the circuit is unbroken if no diagnostic trouble code is set, but despite this you cannot know whether the device is doing its job. Use the diagnostic tool to activate it and check that it is working.

Relays are sometimes involved. The control module is only able to check that everything is OK in its own circuit, i.e. via the relay winding. This tells you nothing about how the fuel pump or radiator fan is working.