Car ECU Problems: 8 Most-Common Issues & Solutions

The Engine Control Unit (ECU) controls and monitors the engine in your car. It reads the various sensors, controls the solenoids, and manages the engine while it is running.

If the ECU develops a problem, it can affect how well the engine performs.

Diagnosing an ECU problem can range from easy to difficult, so we’ve gathered the most common issues and their solutions.

This should help you find the cause quickly and get your car running in tip-top shape again.

#1 – Low Engine Performance

The ECU monitors the amount of air entering the engine and then triggers the fuel injectors to add the correct amount of fuel into the intake manifold for the best engine performance.

If the ECU adds the wrong amount of fuel, the engine may run lean (not enough fuel) or rich (too much fuel). Your engine may stumble with little performance when you step on the gas pedal.

How to Fix It

The ECU references tables of information based on sensor input. The amount of oxygen from the Mass Airflow (MAF) sensor is referenced in the air-fuel table, and the corresponding fuel amount is determined from the table.

This fuel amount triggers the fuel injectors to open for a predetermined amount of time. The ECU also receives feedback information from the oxygen sensors after the combustion process to determine if all fuel and air have been used in the combustion process. All of this information is critical to the performance of your engine.

Your ECU must be able to receive every piece of information sent from all the critical sensors, or the engine performance will suffer.

The first step in determining if the ECU has a problem is to rule out the sensors sending poor information.

You can clean the MAF sensor and its housing, and also check the voltage output of the sensor to rule it out as a problem. You can verify the fuel pressure at the engine to determine if the fuel system is causing the problem.

You can also check the voltage output of the oxygen sensors to determine if one of the sensors is causing a problem. Once you rule out the sensors, then you can diagnose if the ECU is the culprit for low performance.

#2 – Your Fuel Economy Degrades

Engine performance and fuel economy can be related problems.

Poor fuel economy occurs because your engine starts using more fuel than required for the performance of the engine.

It can be related to faulty sensors sending incorrect information or no information if the sensor has completely failed. If the ECU doesn’t understand the information relayed from the sensors, it can cause more fuel to be added to the combustion process as a safety precaution to prevent engine and component damage.

How to Fix It

The Engine Control Unit (ECU) on most vehicles is well protected from the surrounding environment and elements. They rarely fail, so the best course of action to determine the cause is to start with the sensors as a potential failure point.

Each sensor can be verified for proper input and output at the wire harness connection. You can also use a hand-held scanner to connect to the ECU and various modules to determine if an error has been detected.

Assuming the sensors are sending the correct information to the ECU, you can replace the ECU to check if the ECU you have is bad. You can also have it reprogrammed by an authorized service center which may correct a programming problem.

Older ECU housings can be opened to verify the condition of the circuit board, and you may find a bad solder connection, a failed capacitor, or corrosion on the circuits that can be repaired.

Newer ECUs are sealed units that aren’t designed to be opened, so reprogramming or replacement may be the only option.

#3 – Engine Ignition Timing Is Out Of Sync

The engine has sensors that detect where the camshaft and crankshaft are in their respective rotations.

The ECU receives signals from these sensors and injects fuel at a pre-determined point in the rotation of the crankshaft and camshaft.

The ECU also sends power to the spark plugs to light the air-fuel mixture in the combustion process. If the ECU doesn’t understand where the spark should be triggered for combustion, the engine may sputter or not run at all.

How to Fix It

Most new cars have a built-in diagnostic network that will illuminate the Check Engine Light (CEL) in the gauges when it doesn’t receive information from the sensors.

If one of the sensors has failed, the engine will default to basic programming that doesn’t adjust for environmental conditions or changing performance demands.

A hand-held scanner can verify if each sensor is working correctly. If the timing is off, it may be the ECU that can’t understand the information the sensors are sending. In that case, the ECU may need to be repaired or replaced.

#4 – The Engine Stalls, Misfires, Or Doesn’t Start

Your engine requires fuel, air, and spark to start and run. You can have all three components in the combustion chamber, but still not start or run well.

Your ECU measures the air coming in, injects the correct amount of fuel, and then sends power to the spark plugs at the right time for ignition.

If your ECU can’t do all three things, your engine may sputter, stall, or misfire. In the worst-case scenario, the engine may fail to start.

How to Fix It

The ECU has pre-determined air-fuel mixture tables based on readings from various sensors. If the ECU can’t access the information correctly, it may stall or misfire.

You may also hear a popping sound coming from the exhaust as the ignition timing is off and the engine is trying to ignite the air-fuel mixture at the wrong time. The easiest way to fix this is by verifying each sensor is generating the correct signal.

You can use a hand-held scanning tool to monitor each sensor output, and if all sensors are good then the ECU is at fault. It may be able to be repaired, but newer ECUs are sealed units that can only be replaced.

#5 – The Engine Won’t Idle When Cold

The ECU adjusts the air-fuel mixture when the engine is cold by adding more fuel until the engine reaches a certain operating temperature.

If the ECU doesn’t add the extra fuel when the engine is cold, the engine may not idle well. It may sputter, fail to start, or you may need to hold your foot on the gas pedal for a few minutes while the engine warms up.

How to Fix It

Your ECU receives signals from one or more temperature sensors. These sensors may be attached to the cooling system to measure coolant temperature, and another sensor may measure the atmospheric temperature surrounding the car. 

These temperature readings indicate the engine is cold and needs extra fuel to idle while it warms up to operating temperature.

If the ECU isn’t receiving the correct sensor output, it may not understand to add the additional fuel. Either the sensor has failed, or the ECU has failed for that function.

You can use a hand-held scan tool to verify the temperature reading at the ECU, and if it is correct, the ECU is not using the information correctly. It needs to be repaired or replaced.

#6 – The Transmission Has Rough Or Irregular Shifting

The Engine Control Unit controls the automatic transmission and its control unit. The ECU tells the transmission when to shift based on the driving style selected.

If the transmission starts to shift roughly between gears, has irregular shifting, or stays in one gear longer than expected, the ECU may be causing the faults.

How to Fix It

In previous cases, the ECU could have been receiving bad information from individual sensors. When the transmission reacts in an odd method, in most cases the ECU is the cause of the problem.

You can use a scan tool to look for faults and error codes with the transmission control unit to ensure it isn’t the cause of the problem.

If the ECU is the true cause, it may need to be repaired or replaced by a qualified service center. Replacement may include programming the new unit with all options on your car.

#7 – Your Car Was Jump Started Wrong

When you connect battery cables to your car to jump start it, there is just a positive cable and a negative cable. In low lighting, two cables are just enough to mix them up and connect the cables opposite of their intent.

When batteries are connected backwards, meaning positive cable to negative battery terminal, it can overload your electrical system for significant damage.

How to Fix It

The overload that can be caused by a battery connected backward can be a mild to wild problem.

You may blow a handful of fuses including the main fuse, or it can overload the entire system of fuses and blow every one of them. Sensitive electronics like the ECU and DME relay can be overloaded before the fuses blow for protection.

You could completely burn the ECU circuit board, the DME relay could prevent the fuel system from working, or the main relay could stop power from being distributed to the car.

You will need to look through every circuit and fuse box to replace the damaged fuses, then check every electrical function on the car to understand what doesn’t work anymore.

It can be a very expensive mistake to replace all components damaged by reversing polarity on the battery.

#8 – The Check Engine Light (CEL) Stays Illuminated

The Check Engine Light (CEL) is a visual indication that a problem has been detected with your car. When the light stays on for no reason, its can be unsettling.

When the light won’t turn off, the ECU is clearly having a problem.

How to Fix It

If the Check Engine Light won’t turn off, the ECU is having a problem that could be an easy fix.

It could mean the circuit in the ECU that triggers the light to illuminate is damaged and receiving power from another circuit. It’s not a critical failure, but it’s a problem that shouldn’t be ignored.

If the ECU can be repaired, it should be repaired. The CEL staying illuminated can mask when a real problem is detected, and the light is truly correct.

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