OBDII Code Guide

OBDII Code Guide

OBDII standardized the codes and emission monitors for vehicles. The structure or nomenclature can be read by technicians by looking at the single letter and four digits to determine what the codes mean.

In 1996, automakers and SAE standardized many functions of the on-board diagnostic system. This historic agreement ushered in the OBDII era and satisfied specific requirements of the EPA’s Clean Air Act. OBDII provided not only a way for the government to ensure vehicles were meeting emission standards, but also allowed independent shops access to vital diagnostic information.

OBDII also standardized the codes and emission monitors for vehicles. The structure or nomenclature can be read by technicians by looking at the single letter and four digits to determine what the codes mean. 

First (System)

The first letter of an OBDII code indicates what system the code covers. It will always be a letter.

P=Powertrain

B=Body

C=Chassis

U=Undefined

Second (Generic or Enhanced)

The second slot in an OBDII code will always be a number that indicates if the code is generic (standardized by SAE) or enhanced and unique to the manufacturer.

0=Generic

1=Enhanced

Third

Even if the code is enhanced or generic, the code will follow specific nomenclature in the majority of cases. 

1=Emission Management (Fuel or Air)

2=Injector Circuit (Fuel or Air)

3=Ignition or Misfire

4=Emission Control

5=Vehicle Speed and Idle Control

6=Computer and Output Circuit

7=Transmission

8=Transmission

9=SAE Reserved

0=SAE Reserved

Fourth

The next number, or letter in some cases, is dependent on the third number. The fourth number is usually focused on a specific component. If you look at P0400-P0499 codes, for instance, the fourth digit indicates the following emissions components:

0=EGR

1=Secondary Air Injection

2=Catalyst System

3=Catalyst System

4=EGR

5=EVAP

6=Fuel Level

7=Exhaust Pressure Sensor

8=Cooling fan/Exhaust

9=Catch-all for emission component and system performance

Some codes may not follow these rules. For example, code P043E is actually for the EVAP system. While these variables tend to be the exception rather than the rule, you really should look up the code by the time you get to the fourth digit. 

Fifth

The fifth number or letter in an OBDII code is dependent on the third and fourth number. For misfires, it can indicate the cylinder. For some catalyst codes, it can indicate the bank of sensor location.

Some codes use letters A-F. These are the exception and usually indicate a performance issue or a circuit reading that is high, low or open.

For example:

P042A=Catalyst Temperature Sensor Circuit

P042B=Catalyst Temperature Sensor Circuit Range/Performance

P042C=Catalyst Temperature Sensor Circuit Low

P042D=Catalyst Temperature Sensor Circuit High

The Future

When SAE wrote the rules of OBDII more than 20 years ago, they wanted to “future proof” the code system. The codes can be expanded to include telematics, electric vehicles and other technologies in the future if the government mandates it or the automakers decide to standardize some systems.

You May Also Like

Rough Running GM Vehicle

This condition may be caused by an internal circuit board fault within the throttle body.

Rough Running, Reduced Engine Power, Service Engine Soon (SES) Light On, P0123 AND P0222

2020-2021 Chevrolet Silverado 2500/3500 w/6.6L 

L8T engine

2020-2021 GMC Sierra 2500/3500 w/6.6L L8T engine

Using a Scope

Ignition coil and plug diagnostics.

Scope-diagnostics
Diagnostic Procedures For Stop/Start Systems

Stop/start technology is more than just a switch wired to the brake pedal and starter.

Auxiliary Cooling Pumps

An auxiliary cooling pump’s primary function is to circulate warm coolant to the heater core.

Battery Management Means Knowing How It Ages

The battery may be fully charged, but you also need to measure how low the voltages go while the engine is cranking.

Other Posts

Detecting Misfire And Knock Conditions

Knock sensors are mounted on the block near the base of the cylinder head.

Mercedes Ignition Coil Pack Diagnostics

Replacing a single coil might not be the best approach to curing an engine of a chronic misfire.

Chrysler’s Totally Integrated Power Module

The TIPM is on the CAN bus, any issues with the communication between the ECM, ABS and BCM can cause problems with the TIPM.

Mercedes EQS Service Opportunities

EQ is the EV brand for Mercedes. It is the equivalent of Audi’s Etron or VW’s iD brands.