All below are true, I would lean toward an O2 sensor. Take it to a independent garage and find out what the codes are for the Check engine light. 1 dirty or clogged air filter element. 2 emissions system not functioing properly . 3 fault in fuel or electrical systems . Have a mechanic check the PCV valve and the injectors, they should better tell you the problem. The first response sounds plausible. The EGR may also be at fault. An Independant garage is the right start to diagnosing it. The "Check Engine" light just means that the engine control computer has found a problem, and that you need to check the detailed diagnostic codes (or have your service shop check them). The bad fuel mileage is probably because the engine control computer is running the engine in a "safe mode" because it's no longer getting enough reliable data to run it normally. I had this same problem, fuel mileage dropped from about 35 mpg to about 25 mpg. In my case, the problem was an oxygen sensor, which had to be replaced. (Your specific problem may be different, although the computer's reaction makes the symptoms the same.) I replaced the sensor, the idiot light is now off, and mileage is back to 35 mpg. I've had two Civics, both of which eventually had the same problem (1988 CRX at 150,000 miles, and a 1997 EX 4-door at 90,000). Both times it was the throttle position sensor. You can buy an OBD-1 reader for $30 - used on older cars A OBD-2 reader cost around $200 They sell them at all autoparts stores. Check engine light comes on for an "emission system" problem only. I'll go along with the EGR, or the O2 sensors as a possiblity. And I agree with the "safe mode" interpretation as to why it runs fine now. Bottom line is get it to an independent repair shop that specializes in this, and have them fix it. Pull the codes from the computer, match the code to the troubleshooting procedure, follow the procedure to find the source. Repair the source, light will go out if that was the only problem. 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. The " check engine light" is by far one of the most misunderstood technological advances by the public. I am sure I will revise this as time goes on, as it is an in-depth understanding for the public. It is a warning light that is illuminated when there is a problem with 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
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