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Catalytic Converters
Mufflers and Tailpipes
Check Engine Light

Your check engine light stays on and when your mechanic runs diagnostic tests nothing shows up you've tried clearingresetting the codes and it comes back on What could cause it to come on?

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Wiki User
June 17, 2013 11:24PM

Either be patient with the mechanic and try hard to understand what he is telling you. Many of my customers jump to the conclusion "that nothing shows up" after explaining what is happening in that particular situation.

There are "monitors" or self tests the computer runs the car through a drive cycle, if a problem occurs, it may not run all of the self tests until that problem is taken care. Therefore, another problem may exist. It is emission related. OR hook up a scanner that is capable of clearing codes, and hope that none are still active. Disconnecting the battery can create other headaches and will not likely solve your dilemma. Best bet is to contact the local snap-on dealer and have him refer you to a known good shop that specializes in this technology-he will know.

The " check engine light" is by far one of the most misunderstood technological advances by the public. This is an needed in-depth understanding for the public. It is a warning light that is illuminated when there is a problem affecting the EMISSION SYSTEM only. Emission system being the pollution control system. Don't get a hard on against it as it is a good thing once you understand it. One point that was brought up a a recent meeting of technicians was that the amount of hydrocarbons is greater when the gas cap is left off than when the engine is running. Hydrocarbons are part of pollution emitted as gasoline evaporates. Going a step farther, one facet of the emission system is the "Evaporative" portion. This is when the fumes from the gasoline are leaking from the system into the outside air. This is one part of the emission system that can trigger a check engine light. I would say that about 7% of the vehicles that have a check engine light are the result of a loose or inadequate gas cap. But understand that many scenarios are possible with the "check engine light" The vehicle's powertrain computer (note that some vehicles have 17 different computers) will run a series of self-tests. They will only run under certain criteria. And they can be vastly different from manufacturer to manufacturer. Some self-tests are not run until preceding ones have run successfully. So if there is a problem in one particular area that is preventing another self test from running, you can have a situation where one problem is fixed, but another still exists. If you fix a problem and drive the car through a drive cycle that sets the monitor (or self test) the light will go off as it passes that criteria that triggered it in the first place. After 1996, the auto industry went to a idea called OBD II (on board diagnostics). This was to get all the manufacturers onto a similar plane for troubleshooting and powertrain control. While they still differ vastly, many corrections and adaptations were made for technicians to better fix the check engine light problems. Prior to this there were so many different and poor troubleshooting data from a check engine light problem that resolving the problem was much more difficult. Many early warning light of this nature were set to illuminate based on mileage. An Oxygen sensor was one of the things that were meant to be replaced when that mileage was hit. This is much like many current "Change oil lights� that are set based on a pre-set mileage.

Yes, a loose gas cap will cause the MIL light to go on, but tighening it will turn it off after a few restarts. Mechanics, even SAE certified mechanics generally do not know how to diagnose a MIL problem. Normally, each MIL code has a diagnostic procedure, e.g. flow charts with diagnostic steps. Dealers have this information but, for obvious reasons, do not share it with the public or with non-dealer repair shops. Nevertheless, the latter type repair shops should not guess at the problem, but should obtain this information rather than use "stab in the dark" techniques. Example: I had a MIL code that indicated High Voltage on a Particular O2 sensor. Sometimes the O2 sensor is the problem, but not always. After (stupidly) replacing the Oxegen sensor multiple times and not solving the problem, the mechanics assumed the problem had to be the computer or something else. after doing research, I learned the a higfh voltage on an O2 sensor means the fuel mixture is too rich. After checking the vacuum ports and air intake and air filter, I discovered a defective air Temp sensor and replaced it, solving the problem. I am really upset with the mechanics for not finding this, and for guessing (not knowing) what the problem really is. Don't believe anything the mechanics tell you, because remember, it's not their money, it;s yours.