Engine light diagnosis is not often quick thing
Q. Will there be a charge to tell me why my "Check Engine Light" is on?
A. Maybe, maybe not! This is a question we get asked a lot when someone has a "Check Engine Light" on. When a certain auto parts chain claims they will diagnose your car for free, realize that what they are doing is reading the codes stored in your computer for free.
This is only the first step in diagnosing why a Check Engine Light might be on. Occasionally it is a code that is fairly cut and dry and you can be confident of what needs to be replaced or repaired just from the code. Most of the time, however, the code points to the area of the engine or emissions system that has a failure. Often these are layered problems with more than one failure, which tends to complicate matters.
Take for example an "Evap Code." This refers to the evaporative side of the fuel system. The onboard computer tests the integrity of the fuel system in your car regularly. So when you get one of these codes, is it a bad gas cap? Is it a leak in a hose? What about one of the solenoids or switching valves? We had one the other day because the gas fill tube was cracked and rusted. This is where a technician has to work through the system to find the culprit using some high-tech equipment and common sense as he goes through it.
We got an Audi in last week with 13 codes stored. This is when a skilled technician has to think through all the processes of how this particular car's systems work and interact.
In this case there were three codes that my technician suspected were causing all the others, so that is where the diagnosis began. It was observed in the scan data that the computer was constantly trying to add fuel and after a thorough inspection of the engine, several vacuum leaks were discovered from various hoses and from a loose throttle valve. One of the stored codes was for a low signal coming from what is called a Mass Air Flow Sensor, or MAF. After testing the voltages to and from the MAF it was determined that the first step in this repair should be the repair of all the vacuum hose leaks, tightening of the throttle valve and the replacement of the MAF sensor.
After the repair the codes were cleared and a long test drive taken while observing the data on the scanner. If down the road any of the other codes come back they would then be handled on an individual basis.
A. you can see by the Audi scenario, it is not always as easy as reading a code and having the answer. That is, however, where the process will start.
• Douglas Automotive is located at 312 S. Hager Ave., Barrington, (847) 381-0454 and 7218B Virginia Road, Crystal Lake, (815) 356-0440. For information, visit douglasautomotive.com. Send questions to underthehood@dailyherald.com.