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.