Check Engine, Service Engine Soon light, OBD II engine trouble codes

Why my Check Engine light comes on?

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All modern vehicles have a computer or ECM (Electronic Control Module) that controls engine operation. The main purpose of this is to keep the engine running at top efficiency with the lowest possible emissions. With today's strictest emission regulations it's not very easy to achieve - the engine needs to be constantly and precisely adjusted according to various conditions such as speed, load, engine temperature, gasoline quality, ambient air temperature, road conditions, etc.
How it works:
There is a number of sensors that provide the ECM with all necessary inputs such as the engine temperature, ambient temperature, vehicle speed, load, etc. According to these inputs, the ECM makes initial adjustments adding or subtracting fuel, advancing or retarding the ignition timing, increasing or decreasing idle speed, etc.
There is a primary (upstream) oxygen sensor installed in the exhaust before catalytic converter that monitors the quality of combustion in the cylinders. Based on the feedback from this oxygen sensor the ECM makes fine adjustment to the air-fuel mixture to further reduce emissions.
There is another, secondary (downstream) oxygen sensor installed after catalytic converter in the exhaust that monitors catalytic converter's efficiency.
Besides, there are few additional emission control related vehicle systems. For example, there is an Evaporative system (EVAP), designed to prevent gasoline vapors from the gas tank from being released into the atmosphere. It also contains a number of sensors and actuators controlled by the ECM.
The ECM has self-diagnostic capability and constantly tests operation of sensors and other components. When any of the sensor signals is missing or out of normal range, the ECM sets a fault and illuminates the "Check Engine" or "Service Engine Soon" light also called MIL (Malfunction Indication Light) storing the corresponding Diagnostic Trouble Code (DTC) in the ECM memory.

Q: What to do if my "check engine" light is on?

A: The simplest way is to visit your local dealer for proper diagnostic. They have all the equipment and information needed to correct the problem. The problem might be even covered by the manufacturers warranty and possibly repaired free of charge.

Q: Is it safe to drive if my check engine light is on?

A: It really depends what code is stored and what caused it. In worst cases driving with check engine light may cause more damage to the vehicle. A car may even stall while driving. If your check engine light came on, I'd certainly recommend to visit your dealer or a mechanic as soon as possible, just to be on a safe side. If the Check Engine light is flashing, this means that the engine computer (ECM) has detected that your engine is misfiring, which could damage your catalytic converter. Have the your vehicle inspected as soon as possible.

Q: Will disconnecting the battery reset check engine light?

A: Disconnecting the battery might reset the check engine light on some cars. However, instead of doing so, I'd recommend to bring your car to a dealer or a mechanic for a proper diagnostic, and here is why:
- not all cars will clear the code after disconnecting the battery
- often, the problem may be actually covered by the warranty and repaired free of charge by your dealer. For example, if you have the code P0420 - Catalyst System Efficiency Below Threshold it's very possible that your catalytic converter is still covered by the original emission warranty and might be replaced free of charge (would cost you close to $1000 otherwise).
- some problems, if not repaired in time may cause a serious damage and more costly repair.
- disconnecting the battery will cause basic settings of the vehicle's computer to be erased (e.g. idle settings, fuel trim settings, transmission shift points, etc.)
- the Readiness code will be erased, which may prevent your car from completing an emissions test. (Readiness code is an indication that certain emission control components of your car have been self-tested)
- the radio, if code-protected, may be locked after disconnecting the battery
- the "check engine" light will come back anyway if the problem still exist.

Q: can I pull the "check engine" code myself?

A: Having an appropriate scan tool or software and some technical knowledge, it's not so difficult to pull a stored trouble code(s).

OBD II connector located on the driver's side under the dashboard

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 Mas 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 circle 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: Code P0171 reads "System too lean (Bank 1)" and 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: Defective or contaminated airflow sensor, intake vacuum leak, dirty fuel filter, etc. There are certain tests to be performed to find the exact cause of the problem. Common problems with Ford engines are defective airflow sensors and vacuum leaks. As of my knowledge, Ford issued Technical Service Bulletin on this problem, you can check it at Alldata DIY.
I'd suggest to visit your local Ford dealer, they will be able to repair the problem properly.

Where to find specific trouble codes and test procedure

There is a website 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 also can find recalls, service bulletins, price for certain parts and labor, and information about how certain vehicle component or system operates. It's very similar to the information system the car dealers use. Whether you have your own small auto repair shop or Do-It-Yourself minded the information they provide would be equally useful.
For more details follow this link: Where to get auto repair manual?

Where can I buy an OBD II scan tool or software

There are many different scan tools and software available on the Internet, from simple OBD II code readers to sophisticated software that can turn a simple laptop to a powerful scan tool. For example:
OBD-2.com - software with special adapter that can turn your laptop into an OBD 2 scanner. I tried, works well.