The engine control unit is one of the most important components of modern vehicles with combustion engines. If this control unit of the engine is defective, malfunctions can quickly occur. In our guide, you will find out what symptoms are associated with defective engine control units and what you need to bear in mind when repairing and replacing the engine control unit.

What are the functions of the engine control unit?

An engine control unit monitors all engine components and controls the combustion process. In this way, the engine control unit ensures clean and fuel-efficient combustion with low exhaust emissions. In its structure, the unit resembles a small computer that records all the vehicle’s data and processes the information. For this purpose, the engine control unit is connected to numerous sensors in the area of the engine and the exhaust system. If the device detects faulty processes, it can issue corresponding error messages.

Causes for defective engine control units

According to the Allgemeiner Deutscher Automobilclub ADAC (German Automobile Club), defects in the engine and engine control category are the second most common cause of vehicle breakdowns registered by the club. But why does such an important component as the engine control unit installed in the vehicle break down so often? There are many reasons for a defective engine control unit.

• Moisture in the control unit
• Ageing process of the installed components
• Short circuit on the engine control unit
• Corrosion on the connectors
• Vibrations in the vehicle

A frequent cause of defective control units is moisture penetration. In this case, moisture penetrates the housing of the control unit and damages the electronics. The penetration of moisture can be related to a specific event or simply happen over time. Then the components inside corrode or a short circuit occurs.

Wear or corrosion on the control unit

The circuit boards and electronic components installed in the engine control unit are subject to a certain ageing process. When the car is in operation, the parts inside can wear out and therefore tend to fail and malfunction. Vibration while driving can also drive wear, as solder joints and components can become loose over time.

The reason for a malfunctioning engine control unit can also be found outside the unit. For example, corroded plug contacts may be responsible for malfunctions, or the control unit may be affected by a short circuit in the connected cables. Therefore, a comprehensive diagnosis is particularly important when a defective control unit is suspected.

Symptoms of a defective engine control unit

If the engine control unit is defective, symptoms of various kinds can indicate the failure. Since the engine control unit no longer functions as intended, various problems usually occur around the engine. Only in the worst case, however, does a defective engine control unit mean that the car can no longer be started at all.

• Engine does not start
• Misfiring while driving
• Engine stutters or runs out of round frequently
• Engine power noticeably decreases
• Engine control lamp lights up
• Exhaust gas values are incorrect
• Fuel consumption increases

Damage to the engine control unit often manifests itself in symptoms such as irregular engine running or misfiring while driving. Due to the fault in the control unit, the regulation of ignition and ignition timing can no longer be carried out correctly. As a result, the engine may run irregularly, stutter suddenly or stop sporadically.

If the engine control unit is defective, a noticeable loss of engine power may be noticeable in addition to an irregular engine run. If the control unit is faulty, the car will sometimes accelerate more slowly and the engine will have poor power. This may be due to the fact that the defective control unit no longer regulates the fuel injection correctly.

Engine check lights up and bad exhaust values

A defective engine control unit can also lead to symptoms such as the engine control light lighting up or increased fuel consumption. If sensor data can no longer be evaluated correctly, the control unit sometimes suspects a fault in the engine control unit. As a result, the engine control lamp lights up or the engine only runs in emergency mode.

If the engine control unit is malfunctioning due to a defect, sometimes too much fuel is injected into the engine’s combustion chambers. The car then consumes more fuel than usual. If, for example, data from the sensors in the exhaust tract are missing, it is also no longer possible to diagnose the exhaust behaviour. Both a poor combustion process and faulty exhaust gas cleaning lead to increased exhaust emissions.

Repair or replacement of the control unit

If the engine control unit is defective, it must be replaced or repaired. The cost of both options may differ depending on the type of control unit. Repair of the engine control unit is only offered by specialised companies. The specialists carry out a comprehensive diagnosis on the engine control unit and go troubleshooting.

For the repair, the engine control unit is disassembled so that all electronic components can be inspected. Special measuring devices also help to diagnose faulty components. All defective parts are then replaced. Due to the high effort involved, a repair of the engine control unit can be associated with high costs.

Changing defective control units in the car

It is particularly convenient and easy to simply replace the engine control unit installed in the car with another one. Unlike the repair of the control unit, the vehicle can be used again immediately. Whether the cost of a replacement part exceeds the cost of the repair depends on the individual case.

The procedure for changing the engine control unit may differ slightly depending on the type of unit. As a rule, the engine control unit is easily accessible in the engine compartment. However, some manufacturers install the control unit elsewhere, or the device is hidden behind a cover.

Replace control unit and buy as spare part

Once the engine control unit has been located, replacing it with the spare part is usually done quickly. To do this, the control unit is removed and disconnected from the connectors. The intact engine control component then only needs to be installed and connected to the connectors. After the replacement, the engine control unit often has to be re-tuned.

Since the purchase of new control units is often associated with considerable costs, the purchase of a used engine control unit can be worthwhile. Also, new control units are no longer available for every vehicle. When buying a used engine control unit, you should make sure that the unit offered fits the respective engine exactly. Comparing the part numbers on the control units often helps here.

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