The Trouble with Diagnostic Trouble Codes

If any one thing has instilled the DIYer with false confidence, it’s the code reader. Sure, it’s a pretty empowering feeling to pick up a code reader from your local auto parts store, hook it up, and get that number (called a diagnostic trouble code or "DTC").

As soon as people have that number though, they assume they know what part they need to replace. Sure, maybe you’ll get lucky, but you could end up replacing a part that didn’t need to be replaced that could lead to replacing more parts and the problem could still exist. As Bret Bodas told The Car Connection in a June, 2012 article, “following code readers alone is a recipe for emptying your wallet and filling up your driveway with problems.”

Obtaining the trouble code is a small part of what a technician does in order to diagnose a problem with your car. To show you what I mean, I asked Jim Taddei, another RepairPal expert ASE Master Technician, for an example.

Let’s say your code reader spits back Code P0135 - O2 Heater Circuit Malfunction (Bank#1 Sensor#1).

First of all, you will need to know the location of this sensor. In this case, Bank #1 is the part of the exhaust system receiving the exhaust from cylinder #1. Sensor #1 is the sensor closest to the cylinder head, commonly found in the exhaust manifold but always before the catalytic convertor. Got it? (If you are following so far, great. As for me, I got lost at Bank#1.)

Here are the reasons why Code P0135 could be set

1. The sensor heater must have electrical power and ground to function, so a blown fuse will cause a lack of electrical power. Damaged wiring or electrical connection problems can also cause a loss of either power or ground.

2. If a software issue is causing the code to set, you will need to update the software.

3. An internal fault with the control module could cause the code to set.

4. The oxygen sensor could cause the code to set.

To sum it up, Code P0135 could mean a blown fuse, damaged wiring, electrical connection problems, a software issue, an internal fault with the control module, or the problem could be with the oxygen sensor. The truth is, 95 percent of the time, it is the oxygen sensor, but if your problem falls within that 5 percent, it could take days for you to diagnose the problem on your own.

The possibility of multiple probable causes is true for literally every DTC retrieved from the on board diagnostic system, so we strongly suggest going to a qualified repair shop that is equipped and trained to diagnose these systems.

In Conclusion 

No matter what your skills are, unless you are a highly trained and skilled mechanic with the right tools and equipment, you should not attempt most repairs at home. You could end up destroying your vehicle, or even worse, injuring yourself or someone else.

Jim Taddei puts it well: "If you have a hard time putting together IKEA furniture, most DIY car repair is probably out of your league." But with the right tools, preparation, education, and time, there are some repairs you can do at home, and it's always a good idea to familiarize yourself with your car. So, stick to the simple things and leave the rest to the pros. Best of luck!