Check Engine Light Diagnostics

Certainly you’ve heard advertisements for $50 computer scanners that you can buy at parts stores that claim to diagnose your check engine light. Sounds a lot better than a $250 diagnostic fee at a shop, right? Well, as with most things, you get what you pay for.

Those inexpensive scanners are not nearly sufficient for diagnosing a check engine light. Sure, they might give you a code, but what does that code tell you? Essentially: nothing. It gives you an area to start with, but it doesn’t tell you which component or components are failing. And that scanner certainly does not give you a specific read-out of exactly what is wrong with the car, as many people would have you believe. So what is really involved in diagnosing a check engine light?

Actually, we do start with scanning for codes, but again, that’s just a starting point. Depending on the code, there are different procedures for tracing down the related systems to pinpoint exactly which component or components is failing. A check engine light could indicate a problem with a gas cap, spark plugs, plug wires, coils, oxygen sensors, catalytic converters, EGR valves, cam sensors, crank sensors, evaporative leaks, etc. It can even indicate a problem with the computer itself, which is very involved to confirm as you must first rule out any other possibilities before buying a new computer (which, depending on the vehicle, can range from $250 to $1000 or more). All of this testing requires time and expertise, so if a shop quotes you a low price for diagnosis, they are probably not probing deeply enough to really assess the problem.

Now here is where the check engine light can affect a smog test. On a 1994/5 or older vehicle, if the light is on during the test, it will automatically fail. The vehicle must be diagnosed and repaired to get the light off in order for it to pass the smog test.

On 1996 and newer vehicles, though, a smog test failure can occur even if the light is off, but has been on recently and a code is stored in the computer. Computer systems on these newer vehicles run self-testing procedures, and it is during those tests that, if the computer picks up a problem, it will flag a code and turn on the light. Generally, once that occurs, the computer stops looking at other systems, so if another problem occurs, it may not recognize it. This is where the diagnostic procedure becomes even more complicated. Diagnostic testing must occur to diagnose what set the light, and then repairs made. However, once that repair is made and the computer is retrained to recognize that it’s fixed, it then starts looking at its systems again, and if another problem was hiding behind the first problem, the computer will now pick up on it and flag another code. It not uncommon for problems to mask each other in multiple layers, such that diagnosis and repairs must also come in layers in order to uncover all the issues. Thus, unless a vehicle passes a Drive Cycle procedure, it’s possible that the light could come back on for another issue after one issue is fixed.

A Drive Cycle is a process of forcing the computer to run through its self-testing procedure in a short amount of time. Depending on one’s driving style and conditions (in town, freeway, warm weather, cold weather, etc), a computer can run it’s own drive cycle in as little as a day or as much as a few months. Obviously, this uncertain timeframe is inconvenient to diagnosis and repair, so we can force the Drive Cycle to occur in about an hour by running it through a series of specific drive conditions in a particular order in order to force the computer to look at all its systems to see if any problems exist or not. If no problems exist, all the “monitors” will pass the test and the light will stay off, and then the vehicle can pass that portion of the smog test. If any of the monitors does not pass, there is a potential underlying problem that could set the Check Engine Light and cause a smog test failure.

Unfortunately the manufacturers – as well as government legislation pertaining to emissions standards – are forcing computer systems to become increasingly complex and difficult to diagnose. So although a $50 scanner might give you a code, it’s not going to be enough to know what to fix. It takes proper equipment and years of diagnostic experience to navigate the increasingly complex computer systems in cars today (indeed, some new vehicles have over 100 computers on board that all communicate with one another…that’s a web of information if there ever was one!). Make sure your diagnostician is being thorough with his (or her) testing procedures, and not just willy-nilly replacing parts based on what codes show on the scanner.