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

Please note that the information below is not intended to substitute for the advice of a professional mechanic or authorized dealer. In this page we only tried to give you a basic idea what's behind the "check engine" light. Don't attempt to repair your car if you don't have proper knowledge and tools, you can be injured and your vehicle could be damaged. Take your car to a dealer or a repair shop.

Why my Check Engine light comes on?

All modern vehicles have a computer or the ECM (Electronic Control Module) that controls the operation of the vehicle powertrain (the engine and transmission). The main purpose of this is to keep the engine running at top efficiency with the lowest possible emissions. With constantly growing demands for better fuel economy and new strictest emission regulations it's not very easy to achieve. The engine parameters need to be constantly and precisely adjusted according to various conditions such as speed, load, engine temperature, gasoline quality, ambient air temperature, road conditions, etc. That's why today's cars have much more electronics than in early days - there is a large number of various sensors and other electronic devices that help the vehicle computer or ECM to precisely control the engine and transmission operation and monitor emissions.
The vehicle computer system has self-testing capability. When the computer senses that there is a problem with some of the components it stores the correspondent trouble code(s) in its memory and lights up the "Check Engine" or "Service Engine Soon" light to tell you that there is a problem and your car needs to be looked at.

The engine computer or ECM

To properly diagnose what is wrong, you need to take your car to a mechanic or a dealer. The technician at the dealership or a garage will then hook up the scanner to the car computer and retrieve the stored trouble code(s). Then he (she) will look it up in the service manual provided by a car manufacturer. The service manual contains the list of possible codes (about few hundreds) and describes what each code means and what needs to be tested. The code itself doesn't tell exactly what component is defective - it only indicates where to look, what engine parameter is out of normal range. The technician will have to perform further testing to pinpoint a defective part.

How this works?

Let's look deeper how all this 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 further adjustments to the air-fuel mixture to 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 vehicle systems related to the emission control. For example, there is an Evaporative system (EVAP), that prevents gasoline vapors inside the gas tank from escaping into the atmosphere. The EVAP system also contains a number of sensors and actuators controlled by the ECM.
The computer or ECM constantly tests operation of all sensors and 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.
The same happens if a mechanical component of controlled system fails. For example, mechanical problem inside the transmission also can turn the "check engine" light on. Even not properly closed gas cap will cause the "check engine" light to come on - the ECM constantly checks if the gas tank is sealed properly.
To sum up, when the "Check Engine" or "Service Engine Soon" light comes on and stays on, there is a problem with your vehicle. This could be a problem with the engine, transmission, or some emission-related component or system.
The stored trouble code can be retrieved with the special scan tool by the technician. The code itself does not tell exactly which part to replace, it only gives a direction where to look for - the technician has to perform certain tests specific for each code to find the exact cause of the problem.

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

A: The simplest way is to visit your local dealer or a repair shop for proper diagnostic. They have all the equipment and information needed to diagnose and correct the problem. The problem might even be covered by the manufacturers warranty and possibly repaired free of charge. Some auto parts stores can also do the "Check Engine" light diagnostic for free.

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 the "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 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.
- 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

It was quite difficult before, since each car manufacturer had different code assignment and different diagnostic connectors and protocols. Luckily, In 1996 in the United States, a Federal Law came into force requiring all US-sold cars to be OBDII (the On Board Diagnostics system version 2) compliant. This means that all cars from 1996 on must be able to be diagnosed with generic OBDII scanner. The diagnostic connector (the picture) is identical on all OBD II cars as well as its location - somewhere around the driver's place. Usually, on the left side under the dashboard. There is a number of scan tools and software available that can scan the vehicle's computer for trouble codes. However, you need to have some technical knowledge to diagnose the problem; the trouble code itself does not tell exactly which part to replace. For example, the code P0401 reads "insufficient EGR system flow", but it could be set for a number of reasons, such as bad EGR valve, clogged EGR passage or, for example, a faulty DPFE sensor (Ford F-150 common problem) - 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 - read below.

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 sophisticated software that can turn your laptop into a powerful scan tool. Prices vary, but for about $150-$200 you can get nice software for your computer with the adaptor that you can connect to your car's OBD connector. Once connected, you can read the trouble code(s) that turned on your "check engine" and reset it if you want. You also can see some of the engine parameters live on your computer screen. OBDII scan tool should work on any US-sold car after 1996. Search Google for 'OBDII scan tool' or 'OBD2 scan tool software'

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 information about how certain vehicle component or system operates. It's similar to the information system the dealers use.
For more details follow this link: Where to get auto repair 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: 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.

Q: I’ve been told that overfilling the gas tank also can cause Check Engine light to come on, is it true?

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.