Joe has a check engine light came on. Since the vehicle still ran, he took it to a part store that checked the codes. The parts man grinned and said you need mass air flow sensor. Without much thought, Joe made his purchase and headed home to fix his car.

After replacing the mass air flow sensor, the light was still on and now the vehicle ran rough and bogged down on acceleration. Another trip to the part store and this time an oxygen sensor code. Well, maybe the oxygen sensor was bad too and just hadn’t showed up yet? The oxygen sensor is replaced and the battery disconnected. Now the light is out but the car idles rough and still bogs down on acceleration. Two days later the check engine light is back on.

A friend tells Joe, his vehicle did the same thing and it was the fuel pump. A new fuel pump is installed but the check engine light is still on, with the same codes. Joe goes to a forum that suggest the power control module (PCM) can cause this problem. Desperate, Joe buys and installs a PCM. The light is still on and the car runs worse than ever. Joe might have done just as well with a diagnostic dice. Changing parts and hoping to fix a check engine light is known as swaptronics.  Ineffective, swaptronics it the MOST expensive way to attempt auto repair.

Unfortunately Joe made fundamental mistakes that would cost him well over fifteen-hundred dollars and a huge amount of frustration. Joe’s mistakes were, taking diagnostic trouble codes (DTC) literally and listening to unqualified advice. For instance a mass air flow sensor out of range code does NOT mean the sensor is bad. Trouble codes are NOT information, they simply an interpretation of information.  A mass air flow sensor could read out of range because of a vacuum leak, or several other things.

Vehicle diagnostics is the most difficult function of auto repair. Quality shops spend far more money on diagnostic equipment, training and information than anything else in the shop.

The reason quality shops invest so heavily in diagnostics is because they realize it is far less expensive than the other options. A proper diagnosis might have gone like this. The code is retrieved and says mass air flow sensor out of range. A check of vehicle fuel trim also shows the PCM is adding 25% additional fuel. Oxygen sensors are reading very lean, near their threshold.

Professional diagnosis eliminates possible causes without changing parts

"Do-it-yourselfers" can fix a number of things, but they must follow a logical approach and seek qualified information. A service manual for the vehicle and basic digital volt-ohm meter are indispensable. Perhaps as valuable is a quality shop that can offer diagnosis when things get too complex.

When problems are fairly obvious and/or possible solutions are low-cost or needed anyway, there is little risk. For instance a simple misfire where the spark plugs are worn and the plug wires are old. Little is lost by a do-it-yourself replacing them and seeing if this fixes the problem. These items were needed anyway and may well fix the problem.

I think many people’s reluctance to pay for diagnosis comes for poor previous experiences. Many people have used dealerships and shops that employ the "swaptronics" approach themselves.

This is where time spent finding a quality shop will pay huge rewards. As with anything else, there are people who can quickly, professionally get the job done and there is everyone else.

Just because a shop is in the auto repair business does NOT mean they are qualified to diagnose problems

Registered visitors are always invited to leave their comments and thoughts by using the form above. If you need to you can login here or register here.

You can also win a free AGCO coffee cup, by reporting any errors you find, with this form.