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How to use a scan tool

Check engine light on – How to use a scan tool and understand trouble codes

When a check engine light goes on, people either freak out or ignore it. The people who ignore the check engine light have been told that it’s just an “emissions thing.” They know it’s going to cost money to get it checked out, so they don’t care. What they don’t understand is that you only get an emissions issue and a check engine light because something is wrong with your engine. It’s either not burning the fuel efficiently, burning too much fuel, or is running lean. Each of these conditions can damage the engine AND cause damage to the catalytic converter, which can cost you big money. So it NEVER pays to ignore a check engine light.

What is a trouble code

An OBDII trouble code consists of a prefix and four numerical digits. All manufacturers have to comply with a universal code protocol. However, in order to accommodate manufacturers’ needs to communicate trouble codes specific to their make, model, or engine, there are two types of code patterns. OBDII trouble codes are either generic or enhanced. A generic OBDII trouble code is a code that’s universal and applies to all vehicles, while an enhanced code is manufacturer and even make and model specific.

All generic trouble codes start with P0 (zero, not capital O).
All enhanced trouble codes start with P1

Next, OBDII trouble codes are divided into categories based on the system involved.

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

So all P01_ _ codes relate to engine management issues
All P02 _ _ codes relate to fuel and air injectors

Starting to make sense?

Now avoid the HUGE Misunderstanding

Computers are smart and getting smarter, but contrary to what most people think, they DON’T tell the mechanic what to replace. They simply report a condition that’s exceeded factory programmed limits. Let’s take a look at one of the most common trouble codes and the cause of most unnecessary oxygen sensor replacements.

P0131 – Oxygen Sensor Circuit Low Voltage (Bank 1 Sensor 1)

This code means the oxygen sensor located on the same side of the engine as #1 spark plug and in front of the catalytic converter is reporting low voltage. If you have your codes read at an auto parts store, they’ll tell you to replace the oxygen sensor. But that’s not what the code says. It only says that the sensor voltage is low. It DOESN’T tell you why it’s low. Replacing the sensor makes as much sense as a doctor telling you that you need a new heart because your blood pressure is low. No sane doctor would do that. And no professional mechanic would automatically assume that the sensor is bad. The only people that do that are DIYers and auto parts store clerks.

That’s why 80% of the oxygen sensors returned as defective from retail auto parts stores are actually good. Several conditions can cause an oxygen sensor to report low voltage. A vacuum leak is the most common cause. Low fuel pressure is another.

Other kinds of trouble codes

Manufacturers have expanded the use of trouble codes beyond emissions related problems.

P – Powertrain codes relate to emissions issues caused by the engine and transmission. All emission codes start with P.

Each manufacturer is allowed to use their own definitions for the following codes.
B – Body codes usually refer to issues with the charging or starting systems
C – Chassis codes usually refer to brakes and stability control systems
U – Communication-bus/network codes refer to problems with the data lines that run between computer modules.

When do codes set?

Auto computers don’t set a check engine light or trouble code until certain test criteria are met. These criteria are called drive cycles. A vehicle can have dozens of drive cycle criteria, one for each system. Each manufacturer decides when to run the test drive cycle. If the computer runs the test and it turns out good, it sets a Readiness Monitor, meaning that system has tested good and can be inspected by an emissions test station. If a drive cycle has not completed successfully, the Readiness Monitor will not set and the emissions testing station in your state can tell right away that your vehicle is NOT ready to be tested. That way, you can never fool an inspector by erasing trouble codes.

For more information on Check Engine Lights, click HERE

For more information on readiness monitors and examples of drive cycles, click HERE

For more information on oxygen sensors, click HERE

To find out which oxygen sensor the code refers to, click HERE

© 2012 Rick Muscoplat

Posted on by Rick Muscoplat

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