How the OBD-II system works !

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

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Where can I buy an OBD II scan tool or software?

There is a number of scan tools and software available on the Internet, from simple OBDII code readers to a sophisticated software that can turn your laptop into a powerful scan tool with live data. Prices vary anywhere from about $30 and up.
A simple code reader does just that, reads the codes. You also can buy a software with the adaptor that will allow you to connect your laptop to your car's OBD II connector. Once connected, you can read the trouble code(s) that turned on your "check engine" light and reset it if you want. You also can see some of the engine parameters live on your computer screen. An OBD II scan tool should work on any 1996 and newer car in US and Canada.

Where to find specific trouble codes and test procedure

There is a website for do-it-yourself people that for a fairly small fee provides instant access to vehicle-specific repair manual. It's called Alldata DIY - I use it quite often and found it very helpful. Besides "Check engine" trouble codes and corresponding test procedure, it also contains all kinds of diagrams (vacuum diagrams, serpentine belt diagrams, wiring diagrams, etc.), repair instructions, specifications, fluid types, maintenance schedule, component location and a lot more. You can check the Technical Service Bulletins that car manufacturers issue for the dealer service departments. The Technical Service Bulletins describe common problems and solutions for a particular car. You also can find recalls, price for certain parts, labor estimates and the information about how certain vehicle components or systems operate. It's similar to the information system the dealers use.

Q: OBD monitor readiness status say not ready, what could be problem?

A: This means that the OBD readiness monitor indicates that the vehicle's computer hasn't completed testing its systems: the Oxygen Sensor and its heater, EGR, EVAP system and the Catalytic Converter. To complete these tests the car needs to be driven, including some acceleration and deceleration, steady highway cruising and stop and go traffic. Sometimes it may takes just a half an hour of driving; sometimes it takes a couple of days. The EVAP and Catalytic Converter monitors are usually the last ones to complete. Once all emission monitors are complete, the OBD readiness monitor changes status as "Completed". 

Q: Can overfilling the gas tank cause Check Engine light to come on?

A: Yes, overfilling the gas tank can trigger the "check engine" light to come on. Modern cars are equipped with an Evaporative System that prevents gasoline vapors from escaping into the atmosphere. When we overfill the gas tank, the excess gasoline can enter the part of the Evaporative system called Charcoal Canister, which is designed to absorb gasoline vapors rather than raw fuel. This can cause some problems with the evaporative system that can trigger the check engine light.
Don't overfill the tank past the first click of the pump. Check your owner's manual.

Q: My car has the code P0133, how can I clear it?

A: Code P0133 reads "Bank 1 Sensor 1 circuit slow response"; meaning the front oxygen sensor (located before catalytic converter) has slow response time to the changes in the air-fuel mixture. This could happen for many reasons, for example: bad oxygen sensor itself, bad or contaminated airflow sensor, intake manifold leak, exhaust leak, problem with the wiring, etc.
Visit your local dealer or a mechanic for proper testing procedure to pinpoint the exact cause.

Q: What does the code P0102 mean?

A: The code P0102 reads "Mass air flow circuit low input". There is a certain procedure to test Mass Air Flow sensor (MAF) for proper operation. When you bring your car to a dealer, they will perform this procedure to check if the sensor is faulty. Mass Air Flow sensor failure is very common.

Q: I got code P1400 on my Ford Pickup truck, what could that mean?

A: The code P1400 reads "DPFE Sensor Voltage Low" (Ford, Mazda). Often it's caused by bad DPFE sensor or clogged EGR passages. Visit your local Ford dealer, they will be able to help you.

Q: What is the code P1000?

A: P1000 is not actually a trouble code. It just means that the Readiness code has not been set yet or in other words, the engine computer hasn't completed testing certain emission control components. If there is no other problems with the car emission system, the code P1000 will disappear after driving for some time (the drive cycle must include idling, stop and go traffic, acceleration and steady cruising).

Q: I have Ford F 150 with codes P0171 and P0174, is it the O2 sensor?

A: the code P0171 reads "System too lean (Bank 1)" and the code P0174 reads "System too lean (Bank 2)". What it means is that the engine is running lean. There are many possible problems that may cause the air-fuel mixture to be lean: A faulty or contaminated airflow sensor, intake vacuum leak, etc. There are certain tests to be performed to find the exact cause of the problem. Common problems with Ford engines are the defective airflow sensor and vacuum leaks. As of my knowledge, Ford issued Technical Service Bulletin on this problem.
I'd suggest to visit your local Ford dealer, they will be able to repair the problem properly.


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