Function

The problem remains the same: if there is no ignition for the petrol engine, there should also be no injection. Otherwise it can endanger the catalytic converter. Here are three possibilities, how the control unit can determine misfire:
- quick query of pre and after catalytic probe,
- highly dissolving capture of the crankshaft rotation,
- measurement in the area of the secondary voltage.

How it works

With the multiple ignition you can see in the third figure still all ignitions together. On top on the first picture one ignition is absent. If you examine the circuit of the single spark ignition coil (see figure 3), you recognise how the control unit can determine a missing ignition. Because one side of the secondary winding (4a) is connected to the control unit, it can determine the voltage drop via the resistance.

The second figure needs additional explanation. Actually, some injections are recorded here, until the control unit finally switched off, recognizing misfires, after 17 seconds. But why do these signals differ so strongly from normal injection signals (see figure 3)? This is, because an oscilloscope leaves out nearly arbitrarily lines if you increase the time basis to be displayed. It has to leave out pixels, even if they are important for the viewer. So, no worry, all injection signals have about the same deviation, it is just not always displayed.

The exhaust gas measurement is interesting in case of an injection shut-off. Of course, the carbon monoxide values instantly decrease and - especially clearly - the hydrocarbon values decrease, too. What happens, however, in the cylinder with the malfunctioning ignition/injection? It works like an undesirable secondary air pump. Undesired, because the lambda oxygen sensor provides wrong values. If the control unit cannot update its values after every ignition individually, it must necessarily go into the emergency running. Nice if your car features two complete systems (see figure 4), e.g., with six cylinders. Then the regulation at least operates for three out of six cylinders.