What is an ECU and what does it do?
ECU stands for Engine Control Unit: the electronic module that manages how the engine runs. Based on the signals from the sensors, it decides how much fuel to inject and when, and on petrol engines it sets the ignition timing too. It also controls the boost pressure, the throttle, the exhaust gas recirculation (EGR) valve and the exhaust after-treatment systems. You will also come across the terms ECM (engine control module) and PCM (powertrain control module); the latter usually means a control unit that manages the gearbox as well.
The ECU's inputs include the crankshaft and camshaft position sensors, the mass airflow sensor or manifold pressure sensor, the coolant temperature sensor, the lambda sensor and the accelerator pedal position sensor. It talks to the other modules (gearbox, instrument cluster, immobiliser) over the CAN bus. The ECU does not control the ABS or ESP, which are separate modules, and it is not the 'trip computer' that shows your fuel consumption on the dashboard.
You will usually find the engine ECU in the engine bay: on the bulkhead, under the scuttle panel or next to the battery. In some cars it sits inside the cabin, under the dashboard. The most common ECUs are made by Bosch (e.g. the EDC family for diesels and the ME/MED family for petrol engines), Siemens/Continental, Delphi, Denso and Magneti Marelli.
Symptoms of a bad ECU - how to tell if your ECU is faulty
- The engine management light comes on with fault codes for several sensors at once, which come back as soon as you clear them, even though the sensors themselves are working.
- The engine cranks but will not start. The starter motor turns over normally, but the ECU is not sending a signal to the injectors, the ignition coils or the fuel pump.
- The engine cuts out while you are driving or once it has warmed up, and then will not restart until it has cooled down. This is typical of cracked solder joints on the circuit board.
- Limp mode and loss of power. The ECU limits the revs and the boost, and the car accelerates very sluggishly.
- Rough running, juddering or a misfire on one cylinder. A damaged output stage in the ECU stops controlling one ignition coil or one injector.
- No communication with the diagnostic tool. The technician cannot connect to the engine ECU, even though the other modules respond.
- The fault comes and goes, e.g. in damp weather or after driving through a puddle. This is how electronics behave once moisture has got in.
ECU or sensor? Faults with similar symptoms
Most of the symptoms listed above are more often caused by something other than the ECU itself. Before you replace it, rule out the most common causes:
- crankshaft position sensor - no start, or the engine cutting out once it has warmed up;
- mass airflow sensor or manifold pressure sensor - loss of power, limp mode, higher fuel consumption;
- weak battery or bad earth - random faults in several control units at once;
- damaged wiring loom - chafed or melted wires near the exhaust and the cylinder head.
A fault on a single sensor usually points to that sensor or its wiring. Signs that point to the ECU are internal module faults, no communication despite a good power supply, or faults on several components at once when their power supply and earths are in order.
Common causes of ECU failure
- Moisture and water damage - water from a leaking scuttle panel, from washing the engine bay or from driving through deep water. Find out more in our guide to flooded engine control units.
- Voltage spikes - jump-starting, connecting the battery the wrong way round, welding with the battery connected, or a faulty alternator voltage regulator.
- A short circuit in an actuator - a shorted ignition coil or injector, or a short in their wiring, can burn out the output stage in the ECU. When you replace the ECU, the short circuit must be fixed as well, otherwise the new module will burn out in the same way. We describe the symptoms in our guide to burnt-out engine control units.
- Vibration and heat - over time, the solder joints on the circuit board crack. The fault usually starts with symptoms that come and go.
Can you drive with a faulty ECU?
In limp mode the car will usually still drive, but with reduced power, so you should still be able to get it to a garage. If the engine cuts out while you are driving, do not carry on: once the engine stops, the brake servo assistance weakens, and on many cars so does the power steering. Do not put off the repair if the engine is misfiring either, because unburnt fuel can damage the catalytic converter.
How to test an ECU - step-by-step ECU check
- Check the battery. A fully charged battery reads around 12.5 V at rest. A weak battery causes random faults in several control units and makes the car hard to start.
- Read the fault codes with a diagnostic tool that can connect to the engine ECU. Ideally, use a tool that supports your make of car, because a basic OBD code reader only sees some of the faults. Write down the codes and their descriptions before you clear them.
- Work out what the faults relate to. Internal ECU faults (e.g. P0601-P0606) point to the module. A fault on a single sensor points to that sensor or its wiring. If several sensors show faults at once, check their shared supply and earth, because one shorted sensor can pull down the supply to all the others.
- Check the fuses and the ECU main relay. A blown fuse or a failed relay causes a no-start and no communication, just like a dead ECU.
- Measure the power supply and earths at the connector. Use a multimeter to check the permanent live feed and the ignition-switched feed (battery voltage, around 12 V). The best way to check the earth is a voltage drop test: with the meter connected between the earth pin in the connector and the battery negative terminal, and the engine cranking, it should show only a small fraction of a volt.
- Inspect the connector and the wiring loom. Look for corrosion on the pins, moisture in the connector, signs of water on the casing, and chafed or melted wires.
- Check the live data. Coolant temperature, engine speed, manifold pressure and accelerator pedal position should all show sensible values. If the sensors are working but the readings jump about or make no sense, the ECU may be to blame.
- Run an actuator test if your diagnostic tool allows it: activate the injectors, the fuel pump, the throttle or the EGR valve. No response from a working component with a good power supply points to the output stage in the ECU.
If the power supply, earths, wiring and sensors are all fine, but the diagnostic tool shows an internal fault or will not connect to the module, the ECU is faulty. To be sure, have the unit tested off the car, on a test bench at a specialist company. Opening the casing yourself usually just causes more damage.
No communication with the ECU - common causes
If the diagnostic tool will not connect to the engine ECU, and other modules in the car report lost communication with it (code U0100), check the following in order:
- the ECU power supply fuse and relay,
- the battery and the ECU earths,
- moisture or corrosion in the connector,
- a break in the CAN bus (on older cars, in the K-line diagnostic wire),
- the ECU itself.
Also make sure that your diagnostic tool supports the ECU in that car. A basic code reader will often fail to connect to a module that is working perfectly.
Fault codes that point to a faulty ECU
Generic OBD codes that indicate a fault in the ECU itself:
- P0601 - memory checksum error,
- P0602 - control module programming error,
- P0603 - keep alive memory (KAM) error,
- P0604 - RAM error,
- P0605 - ROM error,
- P0606 - ECU processor fault,
- U0100 - lost communication with the engine ECU (logged by another module).
Car manufacturers also use their own codes, so always read the code description in the diagnostic tool rather than going by the number alone.
ECU confirmed faulty - repair or replace?
Minor electronic faults, such as a cracked solder joint or a damaged transistor, can sometimes be repaired by a specialist workshop, but after water damage, or once the processor and memory have burnt out, replacement is the only option. We look at the costs, and at whether a repair is worth it, in our guide to ECU repair and refurbishment.
At Auto24Parts we mainly sell used original engine control units. Before dispatch, every ECU is checked on a test bench: electrically, electronically, visually and mechanically. Used parts come with a 6-month warranty (see our warranty and returns policy).
A used ECU holds the previous car's data in its memory, including the immobiliser data, so it has to be matched to your car before it is fitted. You can buy a Locked ECU (for a garage that will program it itself), an Unlocked ECU in one of several variants, or a Plug & Play Full Set with the immobiliser components. Not every variant is possible for every ECU: it depends on the full part number, the manufacturer, the car model and the immobiliser system. For cloning, you need to send us your old ECU. Preparing an Unlocked ECU adds around 1-2 working days to your order. We explain the details on our ECU preparation options page.
Most searched-for makes: engine control units for Ford, Volkswagen, Toyota and Nissan. To find out how to choose the right ECU and what a replacement with coding costs, see our guide to replacing an engine ECU - cost, coding and choosing the right unit.
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 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; preparing an Unlocked ECU adds around 1-2 working days. 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
Can you test an ECU with a multimeter?
Only partly. With a multimeter you can measure the power supply, the earths and the continuity of the wires in the loom. What a multimeter cannot check is the inside of the ECU: the processor, the memory and the output stages. For that you need a diagnostic tool or a test bench.
Will the car start straight away after an ECU replacement?
Only if the ECU has been prepared for that car: supplied as an Unlocked ECU in the right variant, cloned from your old ECU's data or bought as a Plug & Play Full Set. A Locked ECU has to be programmed by a garage first, otherwise the immobiliser will not let the engine start.
Can every ECU be cloned?
No. Whether cloning is possible depends on the manufacturer and the full ECU part number. Before you buy, send us the numbers and we will check whether your ECU can be cloned.
Does the replacement ECU need the same numbers as the old one?
Yes. Compare the part number and the software number of your old ECU with the numbers in the listing. An ECU with different numbers may not work with the engine and the rest of the car, even if it fits the connector.
How much does an ECU cost?
The price depends on the ECU part number and the car model, and on top of that you need to allow for preparation or coding, and fitting. You will find current prices in the shop categories, and repair costs in our guide to ECU refurbishment.
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, and the exact shipping cost is shown at checkout. Preparing an Unlocked ECU adds around 1-2 working days.
