How the OBD-II system works !

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How the OBD-II system works and FAQ !!


All modern cars and trucks have a computer (in the photo) that controls the operation of the engine, automatic transmission and emission control systems. This computer is usually called the PCM (Powertrain Control Module) or the ECM (Electronic Control Module). The main purpose is to keep the engine performing at the most efficient level while keeping the exhaust emissions low. To achieve this, the PCM constantly adjusts the engine and transmission parameters according to the speed, load, engine temperature, gasoline quality, ambient air temperature and other conditions. We usually don't notice how it works, but from the moment you turn the ignition on, everything is controlled by the computer. On some cars even after the car is shut off, the PCM runs some tests to check some of the components.
All passenger cars and trucks sold in North America since 1996 are OBD-II compliant. AnOBD-II or On-Board Diagnostics version II standard among other things requires the vehicle's computer system to have a self-testing capability. The PCM constantly monitors all the sensors and periodically tests electronic components and emission control systems. When the PCM detects a problem with some of the electronic components or the signal from one of the sensors is out of normal range, it turns the Malfunction Indicator Light or MIL on. At the same time, it stores the correspondent diagnostic trouble code (DTC) in its memory. For some problems, the PCM also stores the freeze frame of the engine parameters at the moment the fault was detected. Read more about the freeze frame below.
Scanning the car computer for check engine codes
Once you take your car to a dealer or repair shop, a technician will hook up the scanner to the car computer and retrieve the stored diagnostic trouble code or codes, as well as the freeze frame. Then, he (she) will look the code up in the service manual provided by a car manufacturer. The service manual contains the list of diagnostic trouble codes (about few hundreds) and describes what each code means, how to troubleshoot it and what parts need to be tested. The diagnostic trouble code itself doesn't tell exactly what component is defective - it only indicates where to look, what engine parameter was out of normal range. The technician will have to perform further testing to pinpoint a defective part. Once the repair is completed, a technician will reset the "Check Engine" light. If you have necessary skills, a good scan tool and the repair manual, you can diagnose the problem yourself. Read more about this below.


Q: What to do if my "Check Engine" light comes on?

A: If the "Check Engine" light came on soon after a fill-up at a gas station, check if your gas cap is closed properly. If the gas cap wasn't on tight, this could be the problem. Secure the gas cap properly and if there is no other problems, the "Check Engine" light should clear itself out in a few days of driving. If the gas cap was tight, there is some other problem. Have your car computer scanned to see what is wrong. It could be some minor problem, but it also could be something more serious that can cause more damage to your vehicle if not repaired in time. Some garages or parts stores often offer to scan your computer for free. Others will charge you a diagnostic fee. If your car is fairly new, I'd advise to visit your dealer for this, because it's possible that the problem is still covered by the original warranty and can be repaired for free or a minimum charge. Check your owner's manual for more details.

Q: Is it safe to drive with the "Check Engine" light on?

A: It really depends what the problem is, what caused the "Check Engine" to come on. It could be something really minor, but it also could be something more serious that needs to be taken care of as soon as possible. In worst cases driving with the "Check engine" light may cause more damage to the vehicle. Here is a very common example:

Often, due to a bad mass air flow sensor, vacuum leak or some other reason, the air-fuel mixture entering the engine becomes lean or there is too much air and too little fuel. The engine computer tries to compensate by adding more fuel, but it only can compensate within a certain limit. If the engine computer can no longer compensate the lean condition, it turns on the check engine light and store in its memory the corresponding code, typically P0171 - System Too Lean (Bank 1) or P0174 - System Too Lean (Bank 2). If caught in time, this problem may require a simple repair and more troubles could be avoided as the lean air-fuel mixture causes pre-ignition or detonation and this could lead to serious engine problems.

If your check engine light came on, I certainly recommend to have your car checked out as soon as possible to be on a safe side.
If the Check Engine light is flashing means that the engine computer (PCM) has detected that your engine is misfiring. Driving with misfiring engine could damage your catalytic converter, which is a very expensive part. Often the misfiring could be caused by bad spark plugs and wires; if you haven't done a tune-up in a while, it might be a good idea to do it now for a start.

Q: Will disconnecting the battery reset the "Check Engine" light?

A: Disconnecting the battery may reset the Check Engine light in some cars, but it will come back anyway if the problem is still there. Besides, the problem may be still covered by the emission warranty (typically 8 years or 80,000 miles) and repaired free of charge by your dealer. Also, some problems, if not repaired in time may cause more damage and more costly repair.
In addition, the readiness code will be erased, which may prevent your car from completing an emissions test (I/M). The readiness code is an indication that certain emission control components of your car have been self-tested.
In addition, the radio, if code-protected, may be locked after disconnecting the battery.

Q: How long does it take for check engine light to reset?

A: If the problem that caused the check engine light to come on is fixed or no longer exists, the check engine light will turn off. For some faults, it may take just a few minutes of driving; for other problems, it may take a few trips. This is because it takes time for a car computer to re-test all the components. If the Check Engine light doesn't clear itself after a couple days of driving, the problem is most likely still there. 

Check engine codes


The trouble codes on all OBD-II cars are standardized and each code has the same meaning on all OBD-II cars. There could be some minor differences in the way different car manufacturers interpret the same trouble code, but the basic meaning is the same.
A typical OBD-II trouble code starts with a letter that is followed by four digits. The letter "P" stands for powertrain, the letter "B" for body. For example, if the engine cylinder number 2 would misfire, the car computer (PCM) would turn on the "Check Engine" light and store the diagnostic trouble code P0302 in its memory. If you'd connect the scan tool, it would read something like: P0302 - Cylinder 2 Misfire Detected.
Overall, there are a few hundreds trouble codes for the powertrain, but only about 40-50 codes are very common. You can read more what some of the common powertrain codes mean here: OBDII Diagnostic Trouble Codes

Freeze Frame

Freeze Frame
A freeze frame is a snapshot of the powertrain parameters at the moment when the engine computer detected a fault and the trouble code was set. A freeze frame is stored in the engine computer along with the trouble code. A freeze frame may contain valuable information that can help to identify a problem faster. For example, a freeze frame may indicate if a vehicle was cold or fully warmed up or if the fault was detected when the vehicle was idling or driven at high speed.

For instance, if you look at this image, this freeze frame for the code P0116 - Engine Coolant Temperature Circuit Range/Performance indicates that the engine coolant temperature was -40F while the intake air temperature was 84F, which is obviously impossible. The engine temperature should be close to the ambient temperature (intake air temperature) if the car is just started or it should be a lot higher if the engine is warmed up. This means that the engine temperature sensor didn't show the correct temperature. This was most likely caused by either the engine temperature sensor being defective or poor connection at the sensor. You also can see that the car was idling at the moment the trouble code was set (the engine speed showed 756 RPM) and the vehicle was stationary (Vehicle Speed at 0MPH).

Q: Can I retrieve the "Check Engine" code myself?

OBD-II DLC Connector
A: Having an appropriate scan tool or software and some technical knowledge, it's not so difficult to scan your car's computer and retrieve a stored diagnostic trouble code (DTC).
In some older cars the trouble code could also be retrieved by observing the blinking of the check engine or specially-designed light. For example, in older Toyota cars, you'd have to connect certain terminals in the diagnostic connector with the jumper wire and count blinks and pauses. In older Honda cars, it would be a special LED on the control unit that would blink the code. Most of the newer cars require a scan tool to scan the computer and check for DTCs.

Before 1996, different cars had different code assignment and different diagnostic connectors. All US-sold cars after 1996 are required to use a standard OBDII (the On Board Diagnostics system Version 2) diagnostic connector and standardized diagnostic trouble codes. The diagnostic connector or Data Link Connector (DLC) is identical on all OBDII cars after 1996 and it should be located within three feet of the driver. Typically, you can find it on the driver's side under the dash or near the hood release (in the photo).

There is a number of scan tools and software available that can scan the vehicle's computer and retrieve diagnostic trouble codes. However, you need to have some technical knowledge to diagnose the problem. As I mentioned before, the trouble code itself does not tell exactly which part to replace. For example, the diagnostic trouble code P0401 reads "insufficient EGR system flow", but it could be set for a number of reasons, such as a bad EGR valve, clogged EGR passage or, for example, a faulty DPFE sensor (common problem in some Ford vehicles). Once you know the trouble code, there is a specific test procedure to be performed to pinpoint the problem part. Where to find specific test procedure and where to buy a scanner

Pourquoi le témoin "Check Engine" s'allume !

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Le témoin «check engine» de votre voiture s’allume et vous en déduisez aussitôt que le moteur est sur le point de lâcher ? Pas si vite, pas si vite.
Depuis 1996, tous les véhicules sont munis de capteurs reliés à un ou plusieurs ordinateurs. Il sont devenus les yeux et les oreilles de ces derniers. Les capteurs détectent les anomalies sur un véhicule. À chaque anomalie correspond un code. On retrouve des capteurs partout sur un véhicule, du bouchon d’essence aux composants internes du moteur. Une voiture standard est munie d’une vingtaine de capteurs. Les constructeurs en ajoutent sans cesse.
Votre témoin «check engine» risque donc de s’allumer pour toutes sortes de raisons: que ce soit pour un bouchon d’essence mal vissé ou pour un cylindre qui n’allume pas. De là l’importance de faire vérifier le véhicule pour en avoir le coeur net. Une fois fait, il faut demander au mécanicien le code de la signalisation et la nature de l’anomalie. Pourquoi ? Pour savoir quel est le problème, s’il est sérieux ou pas, et s’il est important de faire réparer le plus vite possible si cela survient encore.
Un témoin «check engine» s’allume parfois en raison d’un problème des capteurs eux-mêmes. Par exemple, dans le système d’échappement, la température est tellement élevée que la sonde peut cuire. Le capteur fait défaut et signale alors erronément un problème qui n’existe pas forcément.
Si votre témoin «check engine» s’allume alors que votre véhicule a un comportement normal, il suffit de faire vérifier le code et de s’informer sur la nature de l’anomalie et les différentes solutions qui s’offrent pour résoudre le problème, avec l’estimation des coûts en jeu.
Si vous désirez vendre le véhicule ou si vous devez le faire inspecter, vous n’aurez évidemment pas d’autres choix que de remédier au problème.
En général, sur la plupart des véhicules âgés de 12 ans et plus, le témoin «check engine» s’allume régulièrement. Les capteurs s’usent avec le temps.
Le progrès, on n’a pas fini d’en parler…