It helps to think in terms of kokonaisobjekti : total object (expressed by the accusative) and osittaisobjekti : partial object (expressed by the partitive).
Korjaako insinööri lamppua? present tense + partial object means that the action is in progress, "is fixing"
Korjaako insinööri lampun? present tense + total object means that the action is going to be completed, "will fix"