First, make sure the ECU is to blame
Many symptoms blamed on the engine control unit are actually caused by a weak battery, a blown fuse, a bad earth or a faulty sensor. Before you pay for a repair or buy another ECU, check the power supply and earths at the connector, read the fault codes with a diagnostic tool and inspect the wiring loom. Signs that point to the ECU are internal module faults (e.g. P0601-P0606), no communication with the diagnostic tool despite a good power supply, or faults on several components at once even though their wiring and power supply are fine. We describe the symptoms, and the measurements step by step, in our guide on how to check if your ECU is working.
What does ECU repair and refurbishment involve?
An automotive electronics specialist first tests the ECU in the car or on a test bench, then opens the casing and looks for the damaged component. During refurbishment, the parts most often replaced or repaired are:
- cracked solder joints - they crack as a result of vibration and changes in temperature, and the fault comes and goes, e.g. appearing only once the engine has warmed up;
- output stages - the transistors that drive the injectors, ignition coils or relays; they usually burn out after a short circuit in that component or in its wiring;
- power supply components - voltage regulators and capacitors damaged by a voltage spike;
- corroded connector pins - after minor moisture ingress, before the water has reached the circuit board.
Sometimes the software is at fault rather than the electronics. In that case the specialist reflashes the software, keeping your car's data. After the repair, the ECU should pass a test, and once it is back in the car, the fault codes need to be cleared and the car taken for a test drive.
Which ECU faults can be repaired and which cannot
A repair has the best chance of success when a single, known component is damaged and the circuit board and memory are intact. The chances are slim when:
- the ECU has been flooded - corrosion carries on spreading under the chips even after drying and cleaning (find out more in our flooded engine control unit guide);
- the ECU has burnt out - a voltage spike or a short circuit has damaged the processor, the memory or the tracks inside the multi-layer circuit board (see burned engine control unit - symptoms and causes);
- the same fault comes back after an earlier repair;
- the casing has already been opened and no longer seals properly, so moisture gets inside.
A repair does not remove the cause of the failure either. If the ECU burnt out because of a shorted injector, coil or wire, that short circuit must be fixed before a repaired or replacement ECU is fitted, otherwise the damage will happen again.
How much does ECU repair or replacement cost?
There is no single price, because it depends on the type of ECU, the kind of fault and the car model. When you compare quotes, add up the total cost:
- for a repair - the diagnosis, the repair itself, removing and refitting the ECU, sending it to the specialist and the time the car is off the road;
- for a replacement - the price of an ECU with the same numbers, preparing it for your car or having it programmed at a garage, fitting and adaptation.
A new ECU from the car manufacturer's dealer network is usually much more expensive than a used original unit, some models can take as long as several weeks to arrive, and for older cars it may no longer be available at all. That is why a repair is most often weighed up against a used, tested ECU of the same type. You will find current ECU prices in our Engine Control Units (ECU) category.
ECU refurbishment or a used ECU - comparison
- Car data. After a repair you get back the same ECU with your car's data, so it usually does not need programming. A used ECU holds the previous car's data and has to be matched to your immobiliser.
- Long-term reliability. After a repair, you cannot always be sure that other components on the circuit board have not been weakened. A used ECU has been working in another car, and before dispatch we test it on a test bench.
- Time. A repair means sending the ECU to the specialist, a diagnosis and the repair itself. We dispatch a used ECU from stock straight away, and preparing an Unlocked ECU adds around 1-2 working days.
- When to choose a repair: when the fault is minor and known, the ECU is hard to find or you want to keep the original unit.
- When to choose a replacement: after water damage or a burnout, when a fault keeps coming back, or when you need the car back on the road quickly.
How to choose a replacement ECU
An ECU is matched by the numbers on its label, not just by the car model. Even in the same model with the same engine, ECUs differ in version and software. Write down or photograph:
- the ECU manufacturer's part number - e.g. on Bosch ECUs it starts with 0 261 (petrol) or 0 281 (diesel);
- the car manufacturer's part number (OE) - e.g. for Volkswagen, in the format 03L 906 018;
- the software number, if it is on the label.
The numbers in the listing must match the numbers on your old ECU. An ECU with different numbers may not work with the engine and the rest of the car, even if it fits the connector. If you are not sure, send us a photo of the label and we will check which ECU fits.
Why a used ECU will not start the car straight away
The engine control unit works together with the immobiliser, the car's anti-theft system. Its memory holds the data of the car it came from: the immobiliser data, often the VIN and the equipment coding. Once such an ECU is fitted to another car, the immobiliser will not let the engine start until the ECU has been matched to it. This can be done in several ways, depending on the ECU and the car.
Three ways to buy an engine control unit
- Locked ECU - with the previous car's data. This is the option for garages that will read the data, unlock the immobiliser memory and program the ECU to the car themselves. Without that, the car will usually not start.
- Unlocked ECU - prepared by us in one of these variants:
- a) ECU with PIN Code / CAR PASS - the garage enters the code and pairs the ECU with the immobiliser;
- b) Virgin ECU - wiped of the previous car's data; it needs programming after fitting;
- c) Plug & Play - IMMO OFF - with the immobiliser function disabled or modified, only for selected ECUs;
- d) Plug & Play - Auto Programming - after fitting, it takes the data it needs from the car's other modules by itself;
- e) Plug & Play - Clone - it receives the data from your old ECU, so you need to send the old unit to us, and its memory must be readable.
- Full Set (Plug & Play) - the ECU together with the immobiliser components, e.g. the immobiliser module, ignition switch and key, depending on the car. All the parts in the set must be fitted.
Not every variant is possible for every ECU. It depends on the full part number, the ECU manufacturer, the car model, the software version and the type of immobiliser. Preparing an Unlocked ECU adds around 1-2 working days to your order. We explain the details on our ECU preparation options page.
ECU replacement step by step
- Make sure the ECU is to blame: check the power supply, the earths and the fault codes.
- Write down the numbers from your old ECU and choose an ECU with the same numbers.
- Choose how it should be prepared: a Locked ECU for a garage, an Unlocked ECU in the right variant or a Full Set. For cloning, send us your old ECU.
- Fix the cause of the failure, e.g. a short circuit in the wiring loom, an injector or a coil, and check the charging system.
- Disconnect the battery before the replacement, and only unplug and reconnect the ECU connectors with the ignition switched off.
- After fitting, carry out the programming or adaptation required for the chosen variant, clear the fault codes and take the car for a test drive.
Engine control units at Auto24Parts
We mainly sell used original engine control units, and some of them have been refurbished. Before dispatch, we check every ECU on a test bench: electrically, electronically, visually and mechanically. Used parts come with a 6-month warranty. You can return a part within 14 days of receiving the parcel, but ECU preparation services (e.g. IMMO OFF, Virgin, cloning, coding) are non-refundable (see our warranty and returns policy).
Most searched-for makes: engine control units for Ford, Volkswagen, Toyota and Nissan.
Not sure which ECU fits?
Email all the numbers from your old ECU, or a clear photo of its label, to contact@auto24parts.com. We will check which ECU and which preparation variant is right for your car before you place your order. We ship by DHL Express or FedEx Express. Delivery within Europe, including the UK and Ireland, usually takes around 1-2 working days from dispatch. You will see the exact shipping cost at checkout once you have entered your address. As the UK is outside the EU, the recipient may have to pay customs duties and import charges (shipping and delivery).
Frequently asked questions
Is ECU refurbishment worth it?
It is worth it for a minor, known fault, when the ECU is hard to find or when you want to keep the original unit with your car's data. After water damage, a burnout or a fault that keeps coming back, a used ECU with the same numbers usually works out cheaper and is more reliable.
Does an ECU need coding after a repair?
Usually not, because you get back the same ECU with your car's data. Programming may be needed if the software was reflashed or the memory was replaced during the repair. After fitting, clear the fault codes and check the car on a test drive.
Will an ECU from another car of the same model fit?
Only if it has the same part and software numbers. The same model and engine are not enough. Such an ECU also has to be matched to your car's immobiliser, or you can buy it already prepared.
Can I replace the ECU myself?
You can swap the ECU over and reconnect its connectors yourself. Programming, adaptation and coding are done by a garage with the right diagnostic tool, unless you buy an ECU prepared in a variant that does not need this after fitting, or a Full Set.
What is a Virgin ECU?
It is an ECU that has been wiped of the previous car's data. It is unlocked, but not yet paired with any car, so a garage has to program it after fitting.
Can a water-damaged ECU be repaired?
Rarely with lasting results. Water causes corrosion under the chips, and it keeps spreading even after the circuit board has been dried and cleaned. That is why, after water damage, the usual choice is to replace the unit with an ECU with the same numbers.
How long will I have to wait for an ECU?
We dispatch ECUs from stock as quickly as possible. With DHL Express or FedEx Express, delivery within Europe, including the UK and Ireland, usually takes around 1-2 working days from dispatch. Preparing an Unlocked ECU adds around 1-2 working days, and for cloning you also need to allow time to send us your old ECU.
