When that pesky Check Engine light comes on, your heart probably jumps into your throat when your BMW acts like it has no power, quits, or chugs. Generally, when it has no power, it’s in limp-home mode. Get home before you shut it off; it might not start again.
What is limp-home mode?
When the Check Engine light comes on, the computer has found something wrong in one of the systems. If the problem is something that will not cause much additional damage to the engine, the computer goes into limp-home mode.
You could lose power, the engine could chug, or it might have a misfire. When this happens, get home or to the dealership, whichever is closer. If you don’t think it will make it to either place, you have time to find a safe place to pull over.
If you shut the vehicle off, it may not start again. You should not drive it any more than you have to get someplace safe or for repairs. Read more to learn about the BMW Check Engine light.
Diagnosing a Problem When the Check Engine Light Comes on
The Check Engine light is tied to the electronic control unit — the engine’s computer. A BMW could have several computers, including computers for the:
- Transmission
- Heating, ventilation and air conditioning system
- Evaporative system
- Door locks, windows and power mirrors
- Seat, including heated and cooled seats
When the Check Engine light comes on, it means something is wrong with the ignition timing system, the exhaust system or the fuel system. Most of the sensors on the vehicle:
- Tell the computer what the driver is doing, such as pressing on the gas or hitting the brake.
- Tell the computer what the engine is doing so that it can make adjustments elsewhere. For example, the oxygen sensor tells the computer how much excess fuel is in the exhaust, thus allowing the computer to adjust ignition timing and/or fuel delivery.
When the light comes on, that tells you the computer stored a code. The code tells you what is wrong. You must plug a scanner into the on-board diagnostics port (OBD-II) to retrieve the codes. In most cases, the port is under the driver’s side dash.
Blinking vs. Solid Check Engine Lights
If the Check Engine light on your BMW is blinking, you have a serious problem, usually in the emission system. One of the more common problems is excess fuel being dumped into the exhaust. This could overheat the catalytic converter and cause extensive damage.
Pull over as soon as you can, and then call a tow truck to take your vehicle to the dealership.
If the Check Engine light is a steady light, the problem is not a major emergency. You should diagnose the problem or get your BMW to the dealership as soon as possible.
Reasons the Check Engine Light Comes on
Some of the common reasons your BMW’s Check Engine light might come on include:
Oxygen Sensor
If the code comes up as an oxygen sensor, that doesn’t necessarily mean that the oxygen sensor is bad. It is reading an incorrect mixture in the exhaust. It could read a lean mixture (too much air) or a rich mixture (too much fuel). The system will give you a hint in most cases.
If the light comes on for an oxygen sensor code but goes off by itself — it seemingly corrects itself — then the engine is running lean or rich. If the light comes on and stays on and you have a code for the oxygen sensor, the sensor or its wiring is most likely bad.
A bad spark plug, coil pack, or plug wire could cause an oxygen sensor code. If the plug is not firing properly, not all of the gas in the cylinder is burning, which results in a rich condition.
The oxygen sensor tells the computer that too much fuel is going out through the exhaust, so the computer sets a code.
Evaporative System
The evaporative system — often referred to as the evap system — traps and removes vapors that the engine produces when it burns fuel. If you have a loose gas cap, this will turn the Check Engine light on with a code for the evap system.
It might not specify that the gas cap is loose. If you get an evap system code, always check the gas cap for looseness or to see if it needs to be replaced, and then work on the other components of the evap system.
Mass Airflow and Manifold Absolute Pressure Sensors
Your BMW could have a manifold absolute pressure sensor, or it could have a MAP and a mass airflow, or MAF, sensor.
The MAP sensor reads vacuum and positive air pressure and sends that information to the computer.
The computer determines how much fuel you need based on the air going into the intake you need for optimum combustion.
The MAF sensor measures how much air flow is going into the intake manifold and sends that information to the computer. The computer uses that information to determine how much fuel you need for optimum combustion.
In most vehicles, the optimum ratio of air to fuel is 14 parts air to one part fuel. You’ll see it written as 14:1. Some vehicles will narrow that number down, so you might see it as 14.3:1.
If the MAP and/or MAF sensors are not working, the mixture will be too rich or too lean. You might see codes for one or both of these sensors, along with one or more oxygen sensor codes, depending on how many oxygen sensors your vehicle has.
Resetting the Check Engine Light
You can reset the Check Engine light with a scanner or by holding the odometer reset button down while turning the key to the accessory position. The door must be shut, and you must have your seat belt on.
Once you see “Reset” light up on the display, then release the odometer button and briefly hit it once. After that, you should see green LED lights appear. The engine light should be off after the green lights appear.
However, it is not a good idea to reset the codes to turn off the Check Engine light. Resetting the codes does not mean the problem has gone away, and the computer will just turn the light on again. You should always make the repair before you reset the codes.
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