Ford Fiesta 1.25 Petrol Siemens SIM210 ECU (4S61-12A650-EB): Sudden Power Loss, No Restart and Two Cylinders Traced to the Microprocessor

The petrol Ford Fiesta 1.25 on this page was driving fine when its Siemens SIM210 engine ECU failed. It happened on the road, with no warning, and the failure then showed itself in three different ways before anyone knew which part was responsible.

Power lost on the move, then a refused restart

The first sign was a sudden loss of power while the car was being driven. The driver switched the engine off, and when they tried to restart it, it would not start.

Eventually it did start, but it ran rough, firing on only two cylinders. With the car in that state, the owner sent the engine ECU in to us for testing and repair.

Ours is a mail-in service open to owners and garages across the UK, so the unit made the journey on its own and the car did not have to.

One fault behind all three symptoms

Lost power, a refused restart and rough running on two cylinders can read like three separate problems. On this Fiesta they were one. Testing traced them to a single fault inside the engine ECU, or ECM.

That is what this job showed, and it is not a rule. Another car with the same symptoms may have an ECU fault somewhere else in the unit, or a cause outside the ECU altogether; only testing settles which.

The microprocessor was at fault

Every unit we receive is logged, visually inspected and electrically tested, then taken through diagnostics aimed at the fault reported with it. On this SIM210, testing located the fault in the microprocessor, and the unit failed its first run on our test bench.

A finding like that goes to the owner before any chargeable work begins. Nothing is repaired until they have authorised it.

Replaced, reflashed and passed on the second test

We replaced the microprocessor. Changing a component of that kind is microscope work, carried out with temperature-controlled rework equipment.

With the new microprocessor fitted, we reflashed the ECU’s software, then put it back on the bench for a retest.

That retest is the step that decides whether a unit goes home. The first test had found the fault; the second had to confirm the repair had cleared it, on the same bench that had failed the unit. The Fiesta’s ECU passed, and only then did it leave us.

It went back to the owner fully working, with a 24-month warranty on the repair.

Recognising a 4S61-12A650-EB unit

The Ford number on this unit is 4S61-12A650-EB. Sixteen independent sources print it alongside the Siemens number S120977003, and the sources that name the family call it SIM210.

Our own record agrees: the SIM210 table where this pair sits lists 4S6112A650EB with S120977003, and prints the code 6AZB beside the Ford number.

The same Ford number is on record from the Fiesta 1.4 as well as the 1.25, so it is the number on the label, not the engine size, that identifies the unit.

If your Ford carries a different unit, look under Fords fitted with other engine ECUs. For any other make, the place to go is the engine ECU repair service this Fiesta passed through.

If your Fiesta’s engine ECU carries this number, or your car has failed in a similar way, use the Repair Form to say what the car did when the fault struck, then send the unit in for testing.