Most common symptoms of a flooded engine control unit
Water inside the ECU casing causes short circuits on the circuit board tracks and corrosion of the pins. The symptoms of a water-damaged ECU can appear immediately or a few days later, once the moisture starts to eat away at the electronics:
- The engine cranks but will not start - the starter motor turns over, but the ECU is not sending a signal to the injectors or ignition coils.
- A 'Christmas tree' on the dashboard - lots of warning lights suddenly come on at once (engine management light, ABS, ESP), even though those systems are working.
- No communication with the diagnostic tool - the technician plugs the tool into the OBD port but cannot connect to the engine ECU. This is one of the most typical symptoms of a flooded or burnt-out ECU.
- Rough running and limp mode - the engine judders, hesitates, will not rev properly or runs on only some of its cylinders.
- Strange behaviour of ancillaries - the radiator fan runs with the engine cold, the fuel pump runs non-stop once the ignition is on, or the temperature gauge behaves erratically.
- The fault comes and goes - e.g. in wet weather or after the car has been washed.
How does water get into the ECU?
The most common cause is blocked drains in the scuttle panel. Instead of draining away, rainwater collects in the tray below the windscreen, where many cars have their ECU. As the water level rises, an ageing seal on the ECU casing or connector lets moisture inside. Other causes of ECU water damage include cleaning the engine bay with a pressure washer (a powerful jet can get past the connector seal), driving through deep water, and leaks that let water run down the wiring loom straight into the ECU connector.
What to do straight away if your ECU has been flooded
- Do not keep trying to start the engine. Every attempt sends current through a wet circuit board, which means more short circuits and faster corrosion.
- Switch off the ignition and disconnect the battery.
- Remove the water from around the ECU and clear the scuttle drains so that the water does not come back.
- Take the car in for diagnosis and explain what happened. If water has been standing in the ECU, or you can see a green deposit on the pins, be prepared to replace the ECU.
Why repairing or drying a flooded ECU is usually a dead end
Many drivers try to save a flooded ECU: they dry it on a radiator, clean it with isopropyl alcohol or hand it to an electronics technician 'to take a look'. Lasting results are rare, because:
- corrosion keeps spreading - water combined with electrical current quickly destroys the copper tracks, and oxidation carries on even after drying; the fault comes back after a week or a month, often while you are on the road;
- water gets under the chips - under the processor and memory, where it is hard to remove and hard to see;
- diagnosis costs money - opening the unit, ultrasonic cleaning and soldering take a lot of work, and nobody can guarantee that the ECU will keep working for long after water damage.
If water has got inside the ECU, it makes more sense to replace it with a working unit with the same numbers. We explain when repairing the electronics still makes sense in our guide to ECU repair and refurbishment - cost and whether it is worth it.
ECU replacement after a flood: a used original with the same numbers
Instead of paying for uncertain repairs, or for a new ECU from the car manufacturer's dealer network, which is usually much more expensive, you can buy a used original ECU with the same numbers. It is the same part that was fitted at the factory.
- Matched by numbers. The ECU manufacturer's part number, the car manufacturer's OE number and the software number must match the numbers on your flooded ECU.
- Tested before dispatch. We check every ECU on a test bench: electrically, electronically, visually and mechanically. Used parts come with a 6-month warranty (see our warranty and returns policy).
- Matching to the immobiliser. A used ECU holds the previous car's data. You can buy it as a Locked ECU (for a garage that will program it itself), as an Unlocked ECU in one of several variants, or as a Plug & Play Full Set. Not every variant is possible for every ECU, and preparing an Unlocked ECU adds around 1-2 working days. You will find the details on our ECU preparation options page. The whole replacement process is described in our guide to ECU replacement - cost, coding and choosing the right unit.
Where to look for a reliable ECU?
Do not buy a pig in a poke: never buy an ECU without checking its numbers. At Auto24Parts we mainly sell used original engine control units for many car makes:
Most searched-for makes: engine control units for Ford, Volkswagen, Toyota and Nissan.
Not sure which ECU fits?
Email all the numbers from your flooded 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).
Flooded engine control unit - questions and answers
Will drying a flooded ECU on a radiator or with a hairdryer save it?
Unlikely. Even if you evaporate the visible water, the moisture under the chips stays there, and water combined with voltage corrodes the tracks. Such an ECU may work 'for a while', but the corroded connections will fail when you least expect it. Drying only masks the problem - it does not solve it.
Can a flooded ECU be repaired by an electronics specialist?
Technically, sometimes yes, but it is usually not worth it. Repairing a water-damaged ECU requires ultrasonic cleaning, soldering and replacing the corroded components. A skilled electronics technician's labour often costs more than a working used ECU, and corrosion can carry on even after cleaning, so a reliable guarantee is hard to come by.
What are the most misleading symptoms of a flooded ECU?
Above all, no communication with the diagnostic tool. Mechanics then look for the cause in the wiring loom, the fuses and the sensors, which adds to the labour costs. Warning lights for systems unrelated to the engine (e.g. ABS or the airbags) are also misleading: they suggest a fault in other systems, when the real culprit is the engine ECU disrupting communication on the CAN bus.
Do I have to buy an ECU with exactly the same numbers?
Yes. ECU manufacturers (e.g. Bosch, Siemens, Delphi, Magneti Marelli) use different software even in the same car models, depending on the model year and equipment. Compare the part number and the software number on the label of your flooded ECU with the numbers in the listing.
Will the car start straight away after fitting a used ECU?
Only if the ECU has been prepared for your car: supplied as an Unlocked ECU in the right variant, cloned from your flooded 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.
Why choose a used original instead of a cheap replacement?
For engine ECUs, there are practically no cheap aftermarket replacements. The alternative is a new ECU from the car manufacturer's dealer network, which is usually much more expensive and sometimes no longer available for older cars. A used original ECU is the same part that was fitted at the factory.
Does diagnosing a flooded ECU at a garage make sense?
If you know the ECU has been standing in water, or you can see a green deposit on its pins, paying for a detailed diagnosis of the ECU itself is usually not worth it. It is worth checking the wiring loom and the connector, though, as the water may have damaged them too.
Does ECU replacement require a visit to a main dealer?
No. An independent garage specialising in automotive electronics, with the right diagnostic tool, can also replace a used ECU and match it to your car.
What should I do with the old, flooded ECU?
Do not throw it away just yet. If its memory can still be read, the data can be transferred to a working ECU with the same numbers (the Plug & Play - Clone variant). The car then keeps its immobiliser data, VIN and coding. For cloning, you need to send us your old ECU.
Can I return the ECU if the fault turns out to be elsewhere?
You can return it within 14 days of receiving the parcel, under the terms set out on our warranty and returns page. ECU preparation services (e.g. IMMO OFF, Virgin, cloning, coding) are non-refundable, so before you order a prepared ECU, make sure the ECU really is to blame.
How can I protect the new ECU from getting flooded again?
Before fitting it, deal with the cause of the previous failure. The most important thing is to clear the scuttle drains, which are often blocked with leaves and dirt. Also check the seal on the housing where the ECU sits, and do not clean the engine bay with a powerful jet of water.
