Error Codes and You - Scan Code Readers and Why You Need One

Likely the most common question asked by mechanics in the Ask a Mechanic forum when dealing with someone having car troubles they need help with is this:

What error codes are set?

This question refers to what error codes the car's computer might be giving. Even if no warning lights, including the Check Engine Light, are on, there may be error codes being tripped. These can shed a lot of light on what's happening in today's modern cars.

Mechanics refer to these codes as OBD codes (on-board diagnostics) as this is the official name given to them as an industry standard. Introduced in 1996, this standard began putting into place the standardized code types and locations present on nearly all cars today. They monitor everything from the engine, its controls, and some parts of the chassis and body as well.

Most engine manufacturers are now using the OBD-2 standard, so the code thrown by a Ford with a specific problem may be exactly the same as that thrown by a Nissan with the same problem. This cannot be guaranteed, however, since the standards are voluntary, so always use the code translation for your specific vehicle.

How OBD-2 Works

OBD-2 codes have four or five digits. Each designates a part of the car.

The first digit will be a letter. "P" means power train (engine, transmission, etc), "B" means body," "C" means chassis, and "U" is undefined.

Next comes the second digit, which will be a 0 or 1. Zero means "generic" and one means "enhanced". These are manufacturer-specific designations, so may mean different things with different vehicles.

The third digit is the sub-system the code references. There are 10 of them, but the last two (9 and 0) are reserved for the Society of Automotive Engineers (SAE) to be used in future releases of the standard.

1 = Emissions management (fuel, air)
2 = Injector circuit (fuel or air)
3 = Ignition problem or misfire
4 = Emission controls
5 = Vehicle speed and idle control
6 = Computer and output circuit(s)
7 and 8 = Transmission

The fourth and possible fifth digit refer to the actual problem and are variable according to the manufacturer and the specific third-digit code being given. So a "1" as the fourth digit could mean "System Too Lean" in an EM (#1) system or "Oxygen Sensor Warning" in an EC (#4). A fifth digit may further narrow this diagnostic.

An EC Reader

Having a scan reader means having access to all of these codes and the diagnostic information they can provide. Mechanics rely on this information to speed up the process of finding the problem and getting it fixed so that they can concentrate on physically repairing the vehicle rather than spending hours doing circuit tests and other things.

The DIY mechanic who does a lot of his or her own work should buy a quality engine code reader. There are many on the market, but the very low cost ones (under $100) are likely to be limited as to what they can do. Repair manuals, both online and off, will usually have code lists so that you can see what the code deciphers to.