FAQ: The Check Engine light and OBD2 Codes


Many people are confused about the check engine light and why it comes on.

Myth 1: The trouble codes will tell you what sensor to replace.

This is an expensive frame of mind. The Check Engine Light directs your attention to a problem. A trouble code description then directs you to a circuit or system. It will not tell you what sensor to replace. It is about as vague as stating that a book is in the automotive section of the library. You now know what section to go to, but no clue where to look yet. That is why there are flow charts or trouble code trees and diagrams to guide you through specific tests to determine what is the problem. Often it is a broken wire, loose connector or some other cause, than the sensor itself.

Myth 2: You can clear the codes by disconnecting the battery.

This is true on pre-96 vehicles and very few, if any, OBD2 vehicles. Some folks will say, “I disconnected the battery for 15 minutes and the light went out, so it cleared the codes”. No, it didn’t. It may have reset the ECM and the light is no longer present, but the code is still there and if the problem has not been repaired, the light will come back on. The next time you have a problem, now you or the mechanic who is working on the vehicle are going to have to contend with that code as well as any other that is present.

Note: There are the rare cases where the manufacturer has made provisions for clearing the codes by disconnecting the battery or removing the ECM fuse. In most cases of post 96 vehicles, that is not the case, as most manufacturers have made the ECU/ECM/PCM retain the information, even in the event of battery voltage loss. In the largest percentage of the vehicles, unless specifically stated by the manufacturer, an OBD2 scantool is required to clear or reset the codes.

Myth 3: When the check engine light comes on, it always means you have to replace something.

One of the first things that needs to be done when diagnosing the check engine light is to clear the trouble codes, road test the vehicle and then recheck the trouble codes. If the codes come back, then start with the lowest number code and go through the flow charts and diagrams.

The last Myth: The Check engine light means an O2 sensor problem.

Anyone who has taken this to be true and has spent quite a bit of money on replacing oxygen sensors knows this is not true.
First, you will not know, nor will anyone else, what the problem is until you have the trouble codes. Even if it is an oxygen sensor code, a lot of times there are other causes for the code to come about.

Vacuum leaks, poor fuel quality, low or high fuel pressure, a compression problem or a plugged catalytic converter could cause it. If the oxygen sensor is bad, then replacing the sensor still does not finish the repair. If an oxygen sensor failed, then there is a problem with the emissions of the engine. Usually when an oxygen sensor fails, it is because it has become contaminated. Contamination caused by an engine that is not running properly. So you will still need to determine where the originating problem came from. More often than not, there will be no trouble codes for that problem and it will not be evident without some specific tests.

Some basic code structure

OBD-II codes consist of a number of parts. Here is a sample code: P0171 Here is a breakdown of what each digit of the code means:

First Character – System

The first character identifies identifies the system related to the trouble code.

  • P = Powertrain
  • B = Body
  • C = Chassis
  • U = Undefined

Second Digit – Code Type

The second digit identifies whether the code is a generic code (same on all OBD-II equpped
vehicles), or a manufacturer specific code.

  • 0 = Generic (this is the digit zero — not the letter “O”)
  • 1 = Enhanced (manufacturer specific)

Third Digit – Sub-System

The third digit denotes the type of sub-system that pertains to the code

  • 1 = Emission Management (Fuel or Air)
  • 2 = Injector Circuit (Fuel or Air)
  • 3 = Ignition or Misfire
  • 4 = Emission Control
  • 5 = Vehicle Speed & Idle Control
  • 6 = Computer & Output Circuit
  • 7 = Transmission
  • 8 = Transmission
  • 9 = SAE Reserved
  • 0 = SAE Reserved

Fourth and Fifth Digits

These digits, along with the others, are variable, and relate to a particular problem. For
example,a P0171 code means P0171 – System Too Lean (Bank 1).

Decoding resources on the web


One thought on “FAQ: The Check Engine light and OBD2 Codes

  1. Ben Stax

    OLder Chevy and GM OBD1 decoding .. only one totally free program running on WIN 98 and on WinXP look for WINALDL install and make your own connector , this is for the old slow pre ’94 160 Baud systems : the connectors available now are not working because they are taking the higher speed baud connecting point . I built my own cable and connected on, an old Serial port from a Thinkpad IBM Laptop as soon as connected and contact switched on it starts to take data .. if you have trouble on that Chevy 90 / 93 ECU you can connect a 10K ohm resistor between A and B connectors , this will instantly have your engine running on 1000 rpm without influence of connected sensors , only the Idle auto regulator valve in the TBI will get your engine at 1000 rpm . in that way you can check the functionality of the IAC valve in the TBI and also try to find that iregular Idle problem provoked by one of the other sensors .I found the settings for my LPG system in this way and also a bad thermostat occasionally playing up , the WINALDL is very useful to find problems in the older 160 Baud ECU’s .You can find all the progs and info on the internet if you search around ,also the ECU’s that are involved useful for that PRE 94 OBD1 system .

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