schematics and diagrams: Check Engine Light On car?

This light title is a bit of a misnomer. It is sometimes also called the "service engine" light, or "fault" light, among other names. The technical name is Malfunction Indicator Light or MIL. The light is indicating that the engine computer has detected a fault in the system. The computer uses a number of sensors to monitor and control what is happening with the engine and perhaps a few other systems in the car. In most cases, a scanner is used to hookup to the computer thru a connector, usually under the dash by the drivers knees. Here is a typical OBD II connector:

If you don't see it by the driver's knees, check behind the glovebox and also in the engine compartment near the relay box. It may be that your car is too old to have an OBD II connector (OBD II became a standard in 1996). Google OBD II and your car to see if you do. In some cases, there are procedures to extract the trouble codes without a scanner, especially prior to 1996. Also, many cars older than 1996 may have an OBD I connector. These systems may also be scanned, but the data may be less useful in many cases.

The scanner will read the fault codes stored by the computer. Many parts chains, such as Autozone, Advance, O'Reilly, or Pep Boys will provide a free scan of your car/truck. Your mechanic may have a more advanced scanner that can troubleshoot each code down to the component failure, but if you want to do it yourself (or perhaps with a little help from FixYa), write down the codes and look them up online. The following page lists the standard codes and their probable causes: http://www.obd-codes.com/trouble_codes/ . In addition to these standard codes, many cars have special codes or a different diagnostic system. If your codes do not match those in the above link, look online for a special set of codes for your vehicle. Autozone.com may have your specific codes in their free repair website.

The codes and causes only give you some ideas for troubleshooting. Let us know if you need help deciphering codes or troubleshooting your codes. When writing to FixYa, please identify your codes and provide information on any symptoms you are experiencing with the car/truck.
Once you have resolved the problem that set the code and the car has had time on the road to retest the affected system, the light will go out by itself. If it doesn't, a return trip to the parts store scanner can erase the codes.

Here are some tips and links for troubleshooting some standard codes:

First, read the code carefully to identify troubleshooting steps. There is often quite a bit of data put into these codes that would help you to avoid making ineffective repairs. In addition, a few troubleshooting steps may help you isolate the problem to the bad component. If you have access to a scanner, you may be able to run additional tests, including reading data directly from sensors and/or actuating electrical components to confirm their functionality.

P0130-P0175: These codes relate to data from the oxygen sensors. There is a tendency out there to lump these codes together and always change these expensive sensors when these codes appear. Many times, the sensor is indeed bad, but many other times it is not.
In particular, the codes P0171-P0175 point more to the air/fuel mixture and are rarely caused by a failed sensor.
The codes P0171 and P0174 are usually due by a vacuum leak caused either by broken/loose vacuum hoses, cracked air intake hoses or leaking intake manifold gaskets.
The codes P0172 and P0175 are usually due to a failed fuel pressure regulator or injectors stuck open. Please know that I do not propose here to replace or repair any items I identify without further troubleshooting.
Many codes in the range P0130-P0170 may be caused by wiring issues. Note that the oxygen sensor wiring sees a harsh environment, because it routes under the car and close to hot exhaust system components. Always check the wiring when you have a trouble code in this range.

P0420-P0423 and P0430-P0433: The downstream O2 sensors are seeing excess fuel in the exhaust while the upstream sensors are not. This usually indicates the catalytic converter is worn out, but may also be caused by a rich mixture that barely passes the limits that would trip the upstream sensor to identify a rich condition.

P0440-P0469 and P0496-P0499: These codes deal with the car's evaporation emission system, which is designed to minimize the release of fuel vapors into the atmosphere. The basic system seals the fuel tank and stores vapors in a charcoal cannister to be released into the engine for burning at an appropriate time. Many modern systems are much more elaborate than what I describe here. A purge or vent valve/solenoid is used to allow the vapors to be drawn into the engine. It should be noted that these faults rarely affect the performance of the engine, except when the purge valve is stuck open or there is a leak between the purge valve and the engine, which will usually result in a slightly lean mixture.
Codes P0455-P0457 identify leaks in the system. Always check your fuel cap first when these codes are present. If the cap has a good seal, check the evap hose on the fuel tank and follow it to the cannister and on to the purge valve. Replace any suspect hoses, and make sure the purge valve isn't stuck open. After repairing an evap system leak, the car will need a day or two to retest itself and clear the trouble code.

Sensors, sensors, and more sensors. Why do I need all these sensors?
While one can argue that designers have gone too far to add conveniences in the design of cars to the detriment of reliability, most of the sensors added to the engines over time have been needed to meet government imposed efficiency and pollution standards.
Mass Airflow (MAF) sensor: this sensor measures the flow of air into the engine. The computer uses this data to calculate the proper fuel flow for the injectors. This is equivalent to the function of the venturies in a carburated engine. The same function can be performed by the combination of pressure sensors inside and outside the engine. These are known as the manifold absolute pressure (MAP) and barometric pressure sensors. While an engine may start with bad airflow data, it will not run effectively without it. Many cars will not even start without MAF data, while bad MAF data will cause lean or rich operation. Due to contamination of the MAF sensor, lean operation is more common as contaminants tend to reduce the signal from the sensor.
The MAP sensor is also useful even in engines that use a MAF sensor. A MAP sensor can be used to detect sudden throttle opening. A throttle position or accelerator position sensor can also be used for this purpose. The computer uses this data to richen the mixure for acceleration, much like the accelerator pump of a carburetor.
The Crankshaft Position sensor is used to tell the computer when piston #1 is approaching top dead center. The computer uses this data to time the ignition system. Data from additional sensors, such as coolant temperature and intake air temperature as well as knock sensor are used to further optimize ignition timing. The knock sensor detects detonation and the computer compensates by retarding ignition timing to prevent engine damage. Crankshaft position data is required for engine operation. Most modern engines will not start without it. In fact, even a loose sensor will often prevent the engine from starting. This type of malfunction may not even set the MIL. Since the crankshaft rotates twice per engine cycle, additional data is needed on valve timing if a distributor is used for spark distribution. This data may be provided by a camshaft position sensor or other device placed in the distributor. In distributorless engines, this extra data is not used, as the computer fires 2 cylinders at the same time, obviating the need to know if the cylinder is on the compression or exhaust stroke.
Camshaft Position Sensor: provides more precise information on valve timing to optimize fuel injection timing. Many engines will run with a failed cam sensor as the computer can use the crankshaft position data to fire the injectors. However, significant fuel efficiency can be gained by the proper use of camshaft position data.

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Car no start:--

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