1991 Mercedes-Benz 350SD Check Engine Light Diagnosis & Testing Cost

The Check Engine Light can come on at anytime and at any vehicle age. It is not uncommon for this to happen while the car is under warranty and at anytime during the remainder of the vehicle's life.

A professional will connect a diagnostic scan tool to read out the diagnostic trouble codes stored in the vehicle. They should check every system in the vehicle as the light can be illuminated from other things like the transmission or ABS system. There can be numerous codes stored in the computer for days, weeks or months so it's important to identify the code that is causing the light to illuminate. The computer can store additional data like when the code was set, how many times it was set, and the sensor reading at the time it was set. All this info is used to start the diagnostic process. Can manufacturers release information called Technical Service Bulletins that may shine a light on why a code was set. Professional mechanics will always check these before diving too deep into any issues that caused the light to come on. Depending on why the code came on, mechanics will test out the sensors or other items like the injectors, fuel pump, spark plugs or just about any other part on the engine. Once a failed part is identified the mechanic replaces the part, resets the trouble codes then drives the car to verify the issue has been resolved.

We recommend that you never replace a part based on only the diagnostic trouble code from the engine computer. Have the system diagnosed by a professional to ensure you don't spend money on parts that aren't needed. When a shop works on your car for this issue ask them to record all the codes stored in the vehicle on the repair order. If they are having trouble diagnosing an issue, ask them if they've checked the Technical Service Bulletins as these can help to identify really hard problems.

There's a common misconception that mechanics can plug a computer into your car and tell you what's wrong. This is only partially true. The data retrieved from the computers give them clues about what system has registered a malfunction, but not what part has failed. You know how sometimes a light in your house stops working, but it can be the bulb, light socket, wiring, switch or just a power outage? Cars are similar...you may get a diagnostic trouble code for the oxygen sensor but the sensor can be OK and the code is set by a bad connector, wiring, exhaust leak or another sensor in the system. This is why it's crucial that a mechanic performs a proper diagnosis to determine exactly why the light was illuminated. Otherwise they will likely replace unneeded parts before identifying the real issue.

Many novice DIY'ers attempt to address this issue by purchasing a diagnostic code reader and replacing the part identified by the code. We receive an abundance of stories where consumers replace multiple parts at a guess without resolving their issue. In some cases they are successful but when it goes wrong it can get expensive very quickly. For these reasons we recommend leaving check engine light diagnosis to professionals.