There’s a common misconception that computers in production cars are a relatively recent phenomenon when, in fact, the concept dates back nearly 50 years. The development of integrated circuits in the 1960s made computer-controlled engine management systems possible by the 1970s.
Of course, today’s electronic automotive control systems are far more complex than the simple setups of the past. Modern vehicles contain dozens of computers, the most important of which is the engine control module (ECM).
What is an ECM in a Car?
Simply put, the ECM is the computer that’s in charge of governing engine operation. The ECM receives input signals from various sensors, processes those signals, then uses the data to manage engine-related output devices (i.e., fuel injectors, ignition coils, etc.).
Much like your home computer, the ECM also stores program instructions in its internal read-only memory (ROM) and random access memory (RAM).
In the past, most vehicles didn’t have any other onboard computers besides the ECM. But today’s models contain dozens of computers, referred to as modules, that communicate with one another over a data network.
It’s worth pointing out that the engine computer isn’t always referred to as an ECM. For example, BMW refers to its engine management system as the digital motor electronics (DME) instead of an ECM.
You’ll also see the engine computer referred to as the powertrain control module (PCM) if the device oversees both engine and transmission operations.
ECM Replacement Cost
If you have a professional replace your car’s ECM, you can usually expect to pay somewhere between $800 and $1,500 to get the job done. Of course, the exact cost will depend on various factors, such as the year, make, and model of your vehicle.
The Most Common Signs of a Bad ECM
Because the ECM is in charge of so many aspects of vehicle operation, it can cause a wide variety of problems when it fails. You’ll find the most common symptoms of a bad ECM listed below.
Note: ECM failure is quite rare. In many cases, when the ECM is having problems, the cause is a software issue rather than an internal hardware concern. Software problems can usually be fixed by reprogramming the ECM rather than replacing it.
Vehicle Won’t Start
An engine needs three primary ingredients to run: adequate spark, a proper air-fuel mixture, and good compression. Because the ECM is responsible for ignition (spark) timing and fuel injector operation, a faulty ECM can easily result in a vehicle that cranks but doesn’t start.
A bad ECM may also result in a no-crank-no-start condition if the module is integrated into the starting and/or anti-theft circuits.
Engine Performance Problems
The ECM manages a wide variety of engine-related components, ranging from emissions equipment to fuel injectors. Depending on how the ECM fails, it can lead to various engine performance problems, including (but not limited to) misfiring, stalling, surging, and poor acceleration.
Illuminated Warning Lights
As you might guess, a faulty ECM can trigger the check engine light—but that’s not all. In some cases, when the module experiences problems, it will turn on additional warnings, such as the charging system and traction control lights.
Vehicle “Not Ready” for Emissions Testing
On vehicles built after 1996, the ECM runs a series of routine system self-tests, referred to as monitors. If you live in a location that requires emissions testing, the ECM must have run all (or almost all) of the monitors or the vehicle will be deemed “not ready” for emissions testing.
A faulty ECM may refuse to run the emissions monitors. What’s more, if the ECM fails to communicate with a diagnostic tool during emissions testing, the test will be inconclusive.
What Causes ECM Failure?
More often than not, software concerns—rather than internal hardware problems—cause ECM issues. Still, although rare, there are instances where the ECM fails, often due to an overload from a shorted component or circuit.
Other common causes of ECM failure include charging system problems and environmental issues, such as moisture and extreme temperatures.
Can I Drive My Car With a Bad ECM?
Can you drive your car with a bad ECM? The answer is—it depends.
It’s rare for an ECM to become a completely nonfunctional brick. Usually, only a portion of the module’s internal program or circuitry becomes compromised, resulting in a problem with a select circuit. If that circuit doesn’t affect how the vehicle runs, you may be able to continue to drive—at least temporarily.
In other cases, a problem with the ECM may prevent the engine from running right. For instance, if the module failure affects the fuel injection or ignition system circuits, the vehicle will either run poorly or not run at all.
Can an ECM be Repaired?
Although most professional repair shops do not fix ECMs, there are some automotive electronics companies that specialize in module repair. Usually, you remove your old ECM, then send it to the electronics company to be fixed (if possible).
Can I Replace an ECM Myself?
Before you attempt to replace an ECM yourself, it’s important to consult a repair manual or repair database to verify the steps involved. While ECM replacement is usually straightforward on vintage vehicles, most models built after 1996 require the module to be programmed with an OEM-level scan tool upon installation.
Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic. The accuracy and timeliness of the information may change from the time of publication.