Mercedes-Benz S400 Check Engine Light is on Inspection at your home or office.

Common reasons for this to happen:

While a Check Engine Light can come on for a number of reasons, in most cases it is due to one of the following issues:

• Loose or damaged gas cap: It’s hard to believe that a gas cap can prompt your check engine light to come on, but it can. Your gas cap acts as the seal for your fuel system and it helps maintain the pressure in your fuel tank. A loose or damaged cap can reduce your gas mileage and increase emissions from your car.

When that annoying yellow light pops on, the first thing you should do is pull over and inspect your gas cap. If it is loose, a quick tighten could very well solve your problem. If it is damaged, head to your local auto parts store for a new one.

Once you have fixed the issue, continue driving to see if the check engine light turns off. If the light stays on, schedule an appointment with a mechanic.

• Faulty oxygen sensor: An oxygen sensor monitors the exhaust for unburned oxygen, which indicates engine efficacy and the proper air-to-fuel ratio. If you have a faulty sensor, it will not provide the correct data to the onboard computer. The majority of vehicles have two to four sensors. The OBD II code will tell the mechanic which one is giving a reading that is outside of its parameters.

Deterioration of the precious metals inside the oxygen sensor affects the sensors’ ability to give an accurate reading. If you put off getting an oxygen sensor replaced, it will eventually damage your catalytic converter, which can be very costly to replace. Your best bet is to call a mechanic and get a proper diagnosis.

• Failing catalytic converter: A catalytic converter helps reduce emissions in the exhaust gases. It converts the carbon monoxide and other harmful gases into less harmful emissions. A failing converter will negatively affect your gas mileage and when the damage becomes severe enough, your vehicle won’t go any faster when you push down harder on the gas pedal.

In most cases, a catalytic converter will not fail if you properly maintain your car. A failing converter can be the result of a faulty oxygen sensor that has not been replaced or any other engine efficacy issue.

A broken catalytic converter will eventually bring your car to a complete stop. Before that, your gas mileage will be horrid, the car will not run smoothly, and the car will not pass an emissions test. A damaged converter should be replaced immediately.

• Faulty mass airflow sensor: This part directs the computer in your vehicle to add the correct amount of fuel based on the air that is coming through the engine. When a mass airflow sensor starts to fail, the car will stall, it will idle poorly, the emissions will go up, and your gas mileage will go down.

An air filter that has never been replaced or improperly installed can sometimes be the culprit when it comes to a faulty mass airflow sensor. Have your air filter checked and replaced on a yearly basis.

While it is possible to drive for months with a malfunctioning mass airflow sensor, it’s not a good idea. Your gas mileage will drop and the car will start stalling frequently. It is best to call a mechanic and have the sensor diagnosed and replaced if necessary.

• Bad spark plugs and wires: A spark plug acts as a seal in the combustion chamber and provides the gap for the spark that causes combustion in the engine.


While newer cars can go 100,000 miles on a set of plugs, older vehicles, especially those built before 1996, require new plugs roughly every 30,000 miles. There is no way to prevent plugs from failing, it just happens over time. If spark plugs are your issue, get them replaced immediately. A mechanic can make quick work of the job and new plugs in most cases are very affordable.