It would depend on what fails, the ECU can substitute default values for most sensors but this will affect driveability.
Other sensors failing, such as the crank sensor, will result in a non-runner.
As for the fault codes being checked at servicing this would depend on several factors....
..if the garage can be bothered...
..if they have the correct diagnostic equipment..
...if they can decipher the fault codes, earlier ECU's do not have definite P codes as used on EOBD systems.