How to Use an OBD-II Scanner: What It Can and Can’t Tell You – Jalopnik

The world of car diagnostics has changed a lot in the past 50 years. In the 1970s and earlier, a mechanic’s most valuable diagnostic tools were usually his eyes and ears. With a few simple tools and a little knowledge you can solve a variety of problems, but if you don’t know what you’re doing, it’s best to leave it to the professionals.

But fast forward to modern times, just about anyone can diagnose problems with their car without the risk of making things much worse. This is where the OBD-II scanner comes into play. An OBD-II scanner is an advanced diagnostic tool that can help you become a heartthrob superhero. Basically, it’s a handy device that simply plugs into your vehicle’s on-board diagnostic (OBD) port, reads any codes currently stored and sends them back to you. You can then inform your technician of the codes and ask about next steps, or you can research the codes yourself and make plans from there.

Diagnosing something is completely different from actually repairing it, you know. Just reading the codes won’t solve the problem, but it won’t make the situation worse either. Doing this will give you a clearer idea of ​​why that pesky warning light is on on your dashboard, but there are limitations to what an OBD-II scanner can do and tell you. Here are some more ins and outs of these very useful, but often misunderstood devices.

For the uninitiated, let’s clear this up first. It’s very likely that the reason you’re reaching for an OBD-II scanner is because a check engine light has appeared on your dashboard. It’s normal to want to understand why it’s there and get it gone as quickly as possible. The check engine light has many likely causes, so it may not be immediately obvious why it suddenly comes on. An OBD-II scanner can help you by reading the exact code, for example P0101, which indicates that your car’s MAF sensor may be faulty.

Another common code is P0420, which relates to the catalytic converter system and, more specifically, the oxygen level being lower than necessary on bank 1. That may sound a little confusing if you’re not familiar with codes or your catalytic converter system, but the scanner can at least give you a starting point for your problem. It also gives you an option to clear the code and remove the check engine light. Just press the button, wait for the scanner to do its work and voila, no more problems. Right?

Wrong. The scanner only told the car that the problem was gone. When you turn the vehicle back on and drive it for a few minutes, the sensors will likely detect low oxygen levels again and send the data back to your car’s ECU, triggering the check engine light. The scanner only reads the code and deletes it, the problem itself remains and so the warning light will remain on until you fix the problem.

As useful as an OBD-II scanner is, it is far from the only tool a diagnostician has at their disposal. Scan tools are useful for diagnosing dashboard warning lights. The lights indicate that a sensor has detected a malfunction somewhere and you can read the corresponding error codes via the scanner. Brilliantly, not all components of your vehicle are connected to sensors, especially if you drive an older vehicle. OBD-II ports were only introduced as a mandatory measure on cars produced from 1996 onwards. OBD-I ports existed before, but not all scanners are compatible with these older ports. Additionally, not all OBD-II scanners can read all types of trouble codes; many basic scan tools can only read the check engine light codes, not ABS or SRS codes.

For example, imagine that your brake discs are severely warped, have a thick lip on them and need to be replaced. There’s no sensor here, and no matter how smart it is, no OBD-II scanner will tell you that your rotors need to be replaced. However, this is unlikely to be a problem as a quick visual inspection is usually all that is needed to diagnose brake rotor problems. However, in other cases it can be more of a headache. Your car may be burning oil, which can be difficult to diagnose, especially if it is due to something like defective piston rings. A problem like this can cause huge problems for the health of your engine, and yet your trusty scanner may have nothing to say about it.

So while your scanner won’t say that your piston rings are shot, it may throw up a few codes that can certainly help you get an idea of ​​what’s going on. For example, if your piston rings are bad, you will likely experience rough running, in addition to unwanted fumes coming out of your exhaust pipe.

When you plug in a scanner and ask it to check for codes, the OBD-II device can throw out codes that highlight faults in certain cylinders, in addition to faulty O2 sensor codes, as these can be easily damaged by oil contamination from faulty piston rings. These codes alone do not indicate that your piston rings have failed, but when combined with blue smoke coming from your exhaust, it is certainly reason enough to believe that they may be the culprit.

In this case, the OBD-II scanner can point you in the right direction, but a full diagnostic assessment is still necessary. To determine whether or not the piston rings are defective, you should perform a compression test. If the PSI is lower than it should be after the test, this may indicate that the piston rings are defective. So while the compression test may reveal the actual fault, the information from the scanner can point you in the right direction.

Buying a used car comes with all kinds of risks, as evidenced by your worst used car buying experiences. This is especially true if you buy from a private seller, because you will have no warranty to fall back on. An OBD-II scanner can be your best friend in figuring out if the seller is hiding any nasty issues to get the sale. Sure, if the check engine light is on, you’ll probably walk away anyway, but what if the code was cleared right before you arrived? Even worse, the seller could have simply put electrical tape over the lamp to hide the fact that it is on.

If it’s the latter, a scanner will read the diagnostic trouble code (DTC) and you’ll know something is hidden. If the code has now been removed, things may be a little trickier as a simple scanner may not detect the code until the light comes back on. This can happen immediately, but sometimes it can take a while, and by then you may have already purchased the car.

More advanced scanners can help you here. They are more expensive, but some can read historical and even cleared codes, giving you a better understanding of the car’s history and health. Not only can these scanners show you if a code has been recently deleted, but many scanners can also show you older codes. So if you see any historical emissions related codes, you can ask the seller what the problem was and how they resolved the problem. They may be able to provide receipts for O2 sensors or a new catalytic converter to support the answers they provide. Ultimately, when buying a used car, the more information you can get, the better.

Saving money by diagnosing your car yourself is all well and good, but it can sometimes be confusing. So it is best to familiarize yourself with what these fault codes actually mean in advance. The codes are usually written in a uniform manner: a letter followed by four numbers. These codes are not random and you can decipher them easily.

The letter always comes first and is C, B, P or U. B stands for bodywork, C for chassis, P for powertrain and U for network communication. This immediately tells you which part of the car is generating the error code. This is followed by a 0 or a 1. The first means it is a generic code, while a 1 means it is manufacturer specific. Knowing this will help you figure out exactly what the code is telling you. For example, if the code here contains a 1 and you drive a Ford, there is no point in reading what someone did to a Honda to fix the code in their car.

The next number indicates the affected system. Examples include 2 for the fuel system and 7 or 8 for the transmission. Finally, the last two numbers indicate the exact problem. For example, a fault code could be P0301, which indicates a fault in cylinder one, or P0304, which tells us that the fault can be found in the fourth cylinder. Keep in mind that while it’s great to self-diagnose and perhaps even repair your car, if it becomes confusing or overwhelming, consulting a professional may be the best option. Misdiagnosing a code from your OBD-II scanner is relatively easy, so a second opinion is always helpful.