Is the check engine light always on? Are you getting poor performance, stalling, or bad fuel mileage?
These problems that seem disjointed may indicate a bigger problem, which is an engine control unit malfunctioning, also known as the ECM (engine control module).
When it fails, things can get out of hand quickly. Understand the signs of ECU failure and how much it will cost you to fix.
What Is an ECU?
The Engine Control Unit (ECU), also known as the Engine Control Module (ECM), is your car’s primary engine management computer. Think of it as your engine’s brain. It processes information from a number of sensors, and controls important engine functions many hundreds of times per second.
Though there are many engine functions controlled by the ECU in an automotive application, modern automobiles have other control modules that control unique systems. Examples are:
- Powertrain Control Module (PCM), which combines engine control and transmission control
- Transmission Control Module (TCM), which handles transmission operations independently
- Body Control Module (BCM) controls all the comfort and convenience features like power windows, door locks, etc.
- Other control modules for: anti-lock brakes, stability control, airbags, climate control, infotainment, adaptive cruise control, other various forms of driver assist technology, etc.
The ECU continuously monitors key engine conditions, using multiple sensors that measure engine speed (RPM), air temperature and pressure, throttle position, oxygen levels in the exhaust, engine temperature, and the specific positions of both the crankshaft and camshaft.
For example, when you press the gas pedal, the ECU immediately calculates the ideal amount of fuel needed for the sensor inputs and makes the precise fuel injection decision,s and calculates the proper spark timing to produce power while maintaining efficiency.
With this continuous process, the engine can immediately adapt to changing conditions from weather, altitude, and driving demands.
ECU Location
Most manufacturers will install the ECU in a protected location to avoid exposure to heat, moisture, and vibration.
The actual components may vary substantially between vehicle makes and models; however, common locations may be found behind the glove box, under a plastic housing within the engine bay, under the dashboard, or within the passenger cabin (typically under a front seat or under the center console).
Common Symptoms of ECU Failure
Although ECU failure is somewhat rare, there are a few critical symptoms that may indicate a faulty engine control unit. However, as many of these symptoms mimic others, accurate diagnosis is critical before spending money to replace this costly part.
#1. The Check Engine Light.
The Check Engine light on your dashboard is an obvious sign of ECU faults. The ECU could have several issues to trigger this light, and many times, the ECU will trigger the Check Engine light once it has recognized issues with its electronic components, circuits, and sensors.
In some cases, the failing ECU can trigger this light without a problem existing. One important thing to remember when diagnosing faulty EGCs is to retrieve trouble codes from the vehicle’s computer with an OBD-II scanner.
The trouble codes should indicate if the problem is with the ECU itself or with something else.
#2. Poor Engine Performance.
For many vehicle owners, one of the first concerns regarding a failing ECU is poor engine performance. Pressing the gas pedal may yield sluggish acceleration, and the engine may not respond very well to throttle inputs.
Poor engine performance often occurs when the ECU either is interpreting sensor data incorrectly or is providing false signals to its fuel injectors or ignition system.
One thing you may notice is that the engine may shake and vibrate, especially under acceleration or extreme load conditions, such as climbing hills. You may also find the engine may be running rough at idle, particularly when adding additional load in the vehicle, such as additional passengers or towing.
All of the performance issues mentioned above will often begin in additional, essentially random ways and exacerbate their performance dramatically.
#3. Engine Running Rich or Lean.
An example would be when the ECU fails, it may lose its ability to create the proper air-fuel mixture to keep the engine running where it needs to be for proper fuel combustion. This may force the engine to either run too rich (too much fuel) or too lean (not enough fuel).
The vehicle can exhibit many symptoms when the engine runs too rich, including poor fuel economy, black smoke in the exhaust, which is always fun, and possibly a strong fuel smell.
The ECU can also decide that the engine runs too lean, which may begin to exhibit weak power, hesitation during acceleration, and possibly stall out.
The symptoms will worsen as the problem persists over time, and indicative evidence that the ECU can no longer calculate or adjust the fuel delivery from the fuel injectors.
#4. Engine Misfires or Stalls.
A failed control module can result in your engine misfiring or stalling sometimes without any predictability.
A misfire caused by a faulty spark plug or faulty fuel injector will generally target specific cylinders, but once we arrive at misfires that are indicative mostly of faulty ECU conditions, it becomes completely random and unpredictable.
You can have the perfect running conditions one minute, and misfires or stalling conditions offered by the ECU fault can occur the next.
The engine can also shake and hesitate between misfiring which adds to the inherent complexity of diagnosing when presented in these circumstances.
A faulty ECU can cause the ECU to provide incorrect timing signals to the ignition system or command fuel injectors to deliver faulty amounts of fuel. These generally become increasingly more problematic when the ECU fails.
#5. Car Won’t Start.
With ECU-related starting issues, you may experience things from long cranking to total no-start issues. At first, you may notice it takes longer to start the engine or that you must crank longer.
As the ECU “degrades” more, your car will crank but not start at all (even though you have fuel, spark, and proper compression).
This all occurs since the ECU is not providing the proper parameters for fuel injection and ignition timing required to start.
Although many problems can cause a car not to start, ECU‐related starting problems often present themselves gradually and with other symptoms from this list.
#6. Erratic Idle Speed Control.
A weak or failing ECU may intermittently not have the capability to control idle speed. You may notice the engine RPMs change randomly at idle while sitting still, or the engine surges or drops at idle.
This issue is mainly apparent when you are at stoplights or sitting in the park. The engine idle may spike to 1500 RPMs from 500 RPMs without any foot pedal input, or drop almost to stall RPM levels.
Since the ECU controls and regulates idle speed based on conditions such as engine temperature, air conditioning load, etc., an irregular idle speed is most often indicative that the computer is not able to process the incoming sensor data properly.
#7. Variable Valve Timing Problems.
If you have a vehicle with variable valve timing, an ECU failure can lead to obvious performance problems directly related to variable valve timing.
Many times, you may notice rough running at different RPM ranges, or decreasing power at certain engine speeds, with different noise characteristics coming from the top of the engine.
The change in noise may sound like the engine isn’t transitioning smoothly between low and high RPM ranges, due to the ECU being unable to properly command variable valve timing adjustments.
Although mechanical issues can also lead to similar performance characteristics, many times an ECU failure will produce a corresponding diagnostic trouble code relative to the variable valve timing system.
Cost of replacing the ECU
Parts Costs
Replacing an ECU when it is faulty is one of the more expensive repairs related to the engine. The total cost to replace an ECU varies based on the vehicle’s make, model, and year, but is certainly more expensive than repairs to weatherstripping in a used beater versus a new luxury car.
You can typically expect the parts cost for the ECU itself to range between $500 and $1600. Certain manufacturer ECUs for luxury and high-performance vehicles fall into the upper portion of the price range, whereas common vehicles usually cost closer to the low end of the range.
Labor Cost
Labor costs will add another $150 – $300 for installation and programming. Most shops will charge for between 1 – 2 hours of labor time for this repair.
Programming or flashing is part of the process. A new ECU needs to be programmed using your vehicle’s VIN and content. Some manufacturers require dealer-only programming, which will cost additional to the total cost of replacement.
Cost Saving Options
You may find less expensive options using refurbished ECUs, since refurbished ECUs generally cost about 40 – 60% less than a brand new unit. Refurbished has plenty of downsides and risks, plus shorter warranty times.
If you decide to pursue a refurbished ECU, you should only purchase from bankable rebuilders that offer good warranties and can provide proper programming.
Warranty Coverage and Repairs
Some vehicles have ECU coverage in their powertrain warranty, which is usually around 5 years or 60,000 miles. If your vehicle is still in the warranty period, you should check with your dealer on coverage and options before spending money on repairs.
While the idea of repairing an ECU rather than replacing it seems desirable, it is not really typical to “repair” an ECU after it has failed.
The exceptions are problems related to the software or if, say, you have a simple configuration issue with the ECU, and the underlying condition was related to needing to reprogram.
ECU Repair (Reprogramming and Flashing)
At times, an ECU that isn’t functioning can actually be repaired through reprogramming. These days, auto manufacturers release software versions routinely in order to gain better performance, correct a bug, or resolve an issue.
This is similar to how, with the case of smartphones, the phone is perfectly fine, but the operating system or software needs to be updated.
When Programming Is Needed
Reprogramming of the engine control module, or other module, could be the answer if your vehicle is exhibiting any of the symptoms:
- Changes in transmission shift timing have recently
- Minor performance issues post-repair, or after your battery was replaced
- Check engine lights, which have specific software-related codes
- Any TSB (technical service bulletin) issued from the factory pertaining to ecu update
- The engine is running slightly rough, after doing other repairs
The Programming Process
ECU programming is done with special equipment and needs a consistent power supply. Most dealerships and a handful of independent shops will be able to perform this service.
In itself, the programming takes no more than 30 to 60 minutes and costs between $150-$300. Once the programming begins, you cannot start the vehicle or disturb the vehicle at all. If the power is cut or an error occurs, then you will corrupt the ECU software and then “brick” the ECU.
The Bad News
Not every ECU issue can be resolved by programming. If there is physical damage to your ECU (For example, due to flooding) or damage to a component (e.g., if the processor has burnt out), reprogramming the ECU is not going to help.
Other manufacturers have limits on what ecu can be programmed only through the dealership.
Reasons for ECU Failure
The primary causes of ECU failure are water, voltage spikes and physical damage.
Water damage can result from a leaky firewall seal, water can enter when flooding occurs, or corrosion from water entering the ECU case and shorting the circuit. In states that are frequently hit by hurricanes – Florida, Louisiana, and Texas – water damage is an all-too-common hazard.
Voltage spikes are more common during a jump start or with a defective alternator. These spikes can result in a burned-out ECU due to damaged soil circuits and other aspects.
Physical damage usually (but not always) comes from engine vibration, an accident, or shoddy installation. Heat can also contribute to physical damage. Engine bay mount ECUs are at the highest risk of damage from heat.