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Diagnosing a BMW Check Engine Light: 7 Causes + What to Do

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An illuminated BMW check engine light indicates several possible issues

But it’s not always obvious what the exact problem might be. Anything from an engine problem or failing catalytic converter to a faulty oxygen sensor could cause it to come on. 

We’ll discuss the common signs, what to do if your BMW check engine light turns on, how to perform a check engine light reset, and more.  

This Article Contains:

Let’s begin. 

What Causes the BMW Check Engine Light to Turn on?

An illuminated check engine light is often ambiguous – it doesn’t always mean an engine problem. For example, it could indicate that the cooling system isn’t functioning like it should, causing the engine to overheat. 

However, if you have an OBD II scanner, you’re one step closer to solving the problem. Most BMW certified new vehicles and pre owned vehicles offer OBD II scanner compatibility. This includes all vehicles in the new BMW IX inventory and the new BMW I4 inventory. 

Here are some specific causes and scan codes to clarify what the issue might be:  

1. Gas Cap Issues

The gas cap helps maintain fuel pressure pressure inside the tank. But this ability is negatively affected by a broken, missing, or loose gas cap. When fuel pressure drops, the Engine Control Unit (ECU) triggers the check engine light and presents a P0440 fault code You may also notice reduced fuel economy. 

Tip: Checking for a loose gas cap is a good starting point for diagnosing your check engine light.

2. Failing Catalytic Converter

The catalytic converter removes carbon monoxide from the vehicle as an essential part of the emission system. 

However, it tends to clog over time, usually due to excess carbon deposits and unburned fuel. This affects your vehicle’s fuel economy and performance, which triggers the check engine light and the P0420 trouble code. 

3. Fouled Spark Plugs or Spark Plug Wires

Spark plugs ignite the air-fuel mixture in the combustion chamber. They use the high voltage running through the spark plug wires from the ignition coil. Eventually, carbon deposits build up on them – sometimes due to too much fuel (unburned fuel) in the mixture. 

When this happens (or when the ignition coil breaks), the spark plugs don’t work effectively and cause engine misfiring, which triggers the check engine light. In certain cases, the spark plug wires can also malfunction. 

Your ECU will register a  P0300 or P0171 fault code if this occurs. 

4. Faulty Oxygen Sensor (O2 sensor)

The oxygen sensor (O2 sensor) is another essential part of the emissions system. It relays information about unburned oxygen levels in the exhaust to the ECU so that it can control the air-fuel mixture. But when the oxygen sensor breaks (often due to long-term exposure to hot exhaust gases), it provides inaccurate unburned oxygen readings. 

Due to faulty oxygen sensor readings, the ECU produces an unbalanced air-fuel mixture in the engine, causing a misfire and an illuminated check engine light. Your ECU will register the P0175, P0713, P0171, or P0162 check engine light codes, depending on the problem. 

5. Faulty Mass Airflow Sensor (MAF)

The mass airflow sensor (MAF) reads the amount of air entering the engine and sends the information to the ECU so it can produce the right air-fuel ratio in the engine. 

How does the MAF break?
Mostly as a result of exposure to dirty air. Your ECU will register a P0101 diagnostic trouble code if this occurs.  

6. Low or Dead Battery 

Your car’s battery is essential for powering several systems, including the car’s computer system. If the computer system doesn’t receive enough power, it may trigger the check engine light. Your ECU will register a P0562 diagnostic trouble code indicating low voltage. 

7. A Vacuum Leak

Your BMW’s vacuum system pushes fumes through the engine. This ensures an adequate air-fuel mixture and reduces harmful emissions. When there’s a leak, too much air enters the air-fuel mixture, causing misfiring and an active or flashing check engine warning light. Your ECU will indicate the P0171 or P0174 check engine light codes.  

So now you know the causes of a BMW check engine light. 
What do you do next? 

What to Do if Your BMW Check Engine Light Turns On?  

Your next course of action depends on how your check engine warning light illuminates. In most cases, you should get a BMW service from a mechanic. However, some situations are more urgent than others.  

1. Check Engine Light Illuminates Under Certain Circumstances

If your check engine light warning light turns on and off, note any changes in vehicle performance. A difference in performance indicates something is likely to fail soon. So drive your car as little as possible and take it to a mechanic immediately. 

2. Check Engine Light Is Constantly On

A constantly lit check engine warning light means a problem needs fixing or you need preemptive maintenance service. As check engine light problems vary in severity, you may not notice a difference in performance. But you should still get a BMW service to avoid additional issues. 

3. Flashing Check Engine Light While Driving

A flashing check engine light is a serious warning sign. In this case, pull over immediately and call a mechanic to ensure your safety. 

4. Check Engine Light Is on with Performance Issues

Consistent lowered performance accompanied by the check engine warning light may indicate your vehicle is in “limp mode.” This is a feature of modern vehicles that protects them from further damage. Get your vehicle towed or have a certified mechanic come to your door, but avoid driving at all costs. 

Your check engine light illuminated, and you took your car to a certified mechanic.
But what should a BMW owner do if the light stays on afterward?

How to Perform a Check Engine Light Reset?

Most BMW check engine lights reset automatically after the problem is addressed. If yours is still on, it may indicate something else needs attention. You can return your vehicle to the mechanic or use the OBD II scanner to check for further problems. 

If you’re certain there isn’t an issue, you can reset the check engine light by disconnecting the negative battery cable for one minute. Then, reconnect it and turn on your vehicle

Perhaps you need to take your vehicle back to the mechanic or are taking it in for the first time. It’s always better for a BMW owner to know the costs involved beforehand.

How Much Does a BMW Check Engine Light Diagnosis Cost?

A BMW check engine light diagnosis ranges from $100-$140. The repair costs vary depending on the severity of the problem and the number of service hours. A gas cap issue is inexpensive, but a catalytic converter could leave you almost $3,000 out of pocket. 

For certified pre owned vehicles, the warranty may cover these costs. However, with non-certified pre owned vehicles, the owner likely pays. This is an important factor when considering BMW certified offers. 

Final Thoughts

Check engine light problems vary in severity, but a serious issue could cause irreversible damage to your vehicle and risk your safety. So it’s best to get it diagnosed as soon as possible. 

Why not protect yourself and your car by having a mobile auto repair service like AutoNation Mobile Service come to your door?

We service all vehicles, including new vehicles, certified pre owned vehicles, and non-certified cars. In addition, we offer service hours seven days a week, easy online booking, and a 12,000 miles | 12-month warranty on all repairs. 

Reach out for a quote today