Code reader or scan tool?
If you want to do your own car repairs, the first tool you’ll need is either a code reader or scan tool. I hear DIYers whine when their check engine light comes on and the auto repair forum experts tell them to get the trouble codes scanned. The posters seem to think that auto experts can tell them what their problem is by guessing which of the more than 1,000 possible trouble codes is stored in their car’s computer.
But here’s the deal; while you can get the trouble code read for free at many auto parts stores, the data those tools can read is very limited because they use cheap code readers, not pro grade scan tools. What’s the difference? Plenty.
Generic versus Manufacturer specific trouble codes
OBDII trouble codes come in many varieties. The U.S. sets the standards for generic codes but the carmakers themselves set the definitions for manufacturer-specific codes. You have to understand how the trouble codes are arranged before you can decide which code reader or scan tool to buy. Here are the prefix letters and what they mean:
P= Powertrain Code
U= Network Code
B= Body Code
C= Chassis Code
Within those categories are generic and manufacturer-specific trouble codes.
If the second character is a “0” the code is a generic code and means the same thing across all makes and models. So a P0171 mean System Too Lean (Bank 1) for every year, make and model.
The third character tells you which vehicle system the code is from.
0 – Fuel and air metering and auxiliary emission controls
1 – Fuel and air metering
2 – Fuel and air metering (injector circuit)
3 – Ignition systems or misfires
4 – Auxiliary emission controls
5 – Vehicle speed control and idle control systems
6 – Computer and output circuit
7 – Transmission
So a P0304 means Cylinder 4 Misfire Detected.
However, if the second character is a “1”, then you’re looking at a manufacturer-specific trouble code. To find the definition of a P1584, you’ll have to know the year, make, model and engine before you can look up the code meaning. And, carmakers can change the meaning for the same code between years and engines.
If the second character is a “2” it’s a generic code that refers to a system that’s not part of the vehicle system setup shown above.
The cheapest code readers and scan only read P0xxx trouble codes. If you buy one of those, you can wind up in a situation where you have a check engine light but the code reader comes up with no codes. That’s because you really have a B, C, or U trouble code or a manufacturer-specific code stored that your code reader can’t read.
What is an OBDII code reader?
You can buy an economy code reader for around $30 that displays only generic powertrain (P) codes but doesn’t give you the definition of the code. Or, you can spend a bit more (around $50) and get a code reader that displays both the code and the definition. The higher end models also display the readiness monitor status. Read this post to learn more about readiness monitors. The bottom line with code readers is that they at least give you a starting point for your diagnosis, but can leave you stranded if you need more data to narrow the root cause.
The cheapest code readers don’t have batteries. That means you can scan for the code and read the trouble code only as long as it’s plugged into the computer. Once you have the code, you’ll have to go to your home computer to look up the code definition. What a pain. But then again, you are a cheapskate. Oh, by the way, once you unplug it, the information is gone. So you’d better bring a pen and paper with you. But if you spend a bit more, the code reader will have a battery that maintains the code information in memory. Dig a little deeper and you’ll get a code reader with a back-lit screen. Dig deeper still and you’ll get a code reader that’ll spit out the code definitions right on the screen. And many of those higher end readers are update-able (as manufacturers develop new codes).
What is an OBDII scan tool
A scan tool costs more because it does more. The most basic scan tools cost around $100 and still limit you to reading generic powertrain (P) codes and the code definition. They also provide some live data. Live data is actual readings from the computer, so you can see what’s happening in real time. The better scan tools display engine temperature, fuel trim, oxygen sensor readings, MAF sensor readings, etc.
Then there are high end scan tools. These units display generic, enhanced, and manufacturer specific powertrain (P), body (B), chassis (C), and communications (U) trouble codes. A scan tool with those capabilities allows you to diagnose ABS, stability control, airbag, and high end automotive accessories like memory seats, cruise control, etc.
What is a bi-directional scan tool?
Factory scan tools not only read data from the computer but also command the computer to perform tasks. One example is the ability to force the ABS system to perform a bleed procedure where it purges air from the ABS control valves. But a bi-directional scan tool can do a lot more. Can’t figure out whether the problem with your auto door lock is due to a bad actuator or body control module? The easiest way to diagnose the problem is to attach your meter to the door lock actuator wires right at the body control module and command a door lock/unlock procedure. If you don’t see voltage, then the body control module isn’t working.
Should you buy a code reader or scan tool?
I believe that if you’re serious about saving money by doing your own car repairs, you need at least a mid-level scan tool. Code readers just don’t give you enough information.
Let’s not kid each other—a code reader is a toy. Sure, it’ll give you the code, but that’s it. If you’re like most DIY’ers, you’ll get the code definition and replace every part mentioned in the trouble code. So if the code says oxygen sensor lean, you’ll replace the oxygen sensor. The parts manufacturer’s love people like you. Every year tens of thousands of perfectly good 02 sensors are replaced just because the trouble code contained the words oxygen sensor. Engine computers are smart, but they’re not that smart. They may tell you that the oxygen sensor is reporting a lean condition. You assume that means the sensor is bad. Most often it’s telling the truth, the exhaust stream really IS lean. What would cause that? Yup, a vacuum leak. But you won’t get to that point until you’re replaced an $80 sensor and wind up with another check engine light. How to get around that? Buy a scan tool that gives you live data and then learn how to interpret the data.
Here’s how live data helps you
When an engine computer sees a lean condition in the exhaust, the first thing it does is add more fuel to the mixture. This is called Fuel Trim. The computer tracks how much fuel it adds or subtracts. It can add or subtract up to 25%. So, if you’re seeing a lean or rich trouble code, the first thing you should do is look at the live fuel trim data. If the computer is adding fuel, that’s an indication of a vacuum leak. If it’s subtracting fuel, that’s an indication of a leaking fuel injector or a fuel pressure problem. Do you see where this is going? A scan tool gives you the data to actually diagnose the problem. THAT’s why you should pony up the money and buy a scan tool with live data. Yes, they’re expensive. But trust me, you’ll waste far more than that by replacing perfectly good parts because you were too cheap to buy a real diagnostic tool in the first place. Let’s see, you replaced both O2 sensors ($160) and the check engine light is still on. And the root problem was caused by a .25 vacuum hose? You could have owned a scan tool for close to that same money.
The most common consumer grade scan tools are made by Actron, AutoXray, Autel, Innova, and Launch.
Don’t buy based on price. You’re buying this tool to help you diagnose and fix the problem yourself so you don’t have to pay a shop. It’s like any other tool purchase—you get what you pay for. Read their specs and MAKE SURE the one you like actually works with your vehicle. Many of the less expensive scan tools do NOT read manufacturer-specific codes–especially on European vehicles. You’ll have to spend upwards of $300-$500 to get that capability. Finally, none of these inexpensive tools will read ABS, AIRBAG, Chassis, or Universal codes. To get that capability, you’ll have to spend about $2,000. Want that? Great. Buy an OTC Genisys scan tool.
© 2012 Rick Muscoplat
Posted on by Rick Muscoplat