Should you follow the error codes from the OBD system? Are car diagnostics accurate?

These questions come to mind whenever you’ve made an unsuccessful repair according to the error code you got from the diagnostics system. 

Luckily, this isn’t a common case but with modern computerized cars, every car owner is bound to get their fair share of this problem.

When and if this happens, this article will provide some information that should help you overcome this problem or at least better understand it.

If you’re not in the mood for reading, you have a YouTube video at the end of this article or on our YouTube channel.



Car diagnostics (better known as the On-Board Diagnostics system) is a system that overlooks all of the main parameters of the car.

These would be vital systems like the fuel injection, air intake, ignition system, brake system, and else.

It consists of one or more Electronic Control Units and an array of sensors that are connected through the wiring installation.

Sensors, which are the „eyes and ears“ of the system, constantly monitor how everything works. If there’s a fault, it will send a signal to the ECU which then sends a warning signal to the dashboard since it’s the most visible place for the driver.

If you want to learn more about dashboard warnings, click here for a separate article on that topic.

Once the error message appears, you’re supposed to hook up an OBD scanner, read out the error code and get a precise and definitive description of the problem.

Essentially, an electronic plug-and-play system that’s meant to solve the problem quickly and precisely. Besides this, it enables almost anyone that has minimal car knowledge to pinpoint the problem.

A far cry from times when the only way to solve a car problem was sheer logic, good knowledge around cars, and previous experience with fixing.

This is a representation of the OBD system, in a nutshell. If you want a more thorough explanation, click here for one provided by Wikipedia.




In order to better understand are car diagnostics accurate, it’s good to know what are some of the most common causes of problems:

  • Nature of the whole system

Electronic assemblies are, by their nature, susceptible to problems, even in the best conditions. They are very sensitive to good electric contacts, a good ground connection, steady voltage outputs, and other vital parameters.

All of these are more or less, weak spots on cars, especially on older ones. Manufacturers do a great job to prevent this and ensure longevity, but nevertheless, it does give in overtime.

Not to mention the harsh environment to which all of the electronic components are subdued.

  • Harsh environment

Extreme heat, extreme cold, moisture, filth, vibrations….these are just some of the elements the electronic components have to withstand and you don’t know which is worse.

Electronics like a stable environment with stable parameters and the car is the exact opposite.

  • Material fatigue

However protected and well made the electronic components are, sooner or later they fall victim to this.

Perhaps the main problem isn’t in the electronic components themselves. They are usually built to last a long time. A good example is the car’s ECU which, if everything is OK, lasts as long as the car.

More problems are caused by things like a bad gasket or seal which can let water or moist in and cause a short circuit.

Other examples are: a loose holder can cause vibrations that may cause damage to the circuits, bad ventilation can cause overheating and the list goes on.

The most common victims of material fatigue in the whole system are sensors as they are the most exposed.

  • Voltage fluctuations

Car diagnostics are highly dependant on a stable electric current. If it fluctuates it can cause all kinds of bad readings and error codes that defy logic.

A very common problem when it comes to determining are car diagnostics accurate. It’s also a sign that something may be wrong with the alternator.

If you want to read more about the most common symptoms of an alternator problem, click here.

  • Bad electric contacts

One single bad connection inside a pin connector can cause a lot of trouble. This is how sensitive the whole system is.

Not to mention more severe reasons like moisture and water which inevitably lead to corrosion over time.

Also, a loose nut or screw holding, for instance, a ground connection, can cause a lot of trouble. You can read more about this problem by clicking here.

Again, car manufacturers do a great job to protect the whole system, but time inevitably takes its toll. This is one of the reasons why these problems are more present in older cars.




So, to finally answer the question: are car diagnostics accurate?

The answer, at least in my humble opinion and experience is: YES!

Most of the time, you can successfully repair the car thanks to the error code you get from a scan tool.

If not the exact reason, you’ll get a good direction towards what system you should look at.

But what about the problems and “false” readings, you’ll ask?

Well, again from experience, lots of these mishaps are not due to the OBD system itself but rather to other factors that influence it. Good examples are the mentioned alternator faults, bad contacts, ground connections, and so on.

Besides this, sensors are considered to cause a lot of problems. But these are car parts like any other that are meant to be replaced after a certain period of time. Many don’t know this and consider this a major fault when in fact it isn’t.

Then there is the question of the quality of parts. A low-quality sensor or component will probably give in a lot sooner than from a renowned manufacturer.

When it does, again people blame the whole diagnostic system rather than the part.

Other major factors are expectations and interpretations. From when the OBD system was implemented, people expect that it will immediately give a magic solution to the problem.

On a car, this is simply not possible and probably never will be.

The truth is that the car is made of thousands upon thousands of parts and various intertwined systems that are monitored by only a handful of sensors, so to say.

Furthermore, the system cannot tell you if there’s a rusty contact or a loose screw somewhere in the system.

Because of this, it’s maybe best to look at the onboard diagnostics as a monitoring system of limited capacity rather than an accurate “fortune teller”.

To be honest, we should be satisfied with this.

If you’ve comprehended the previous two parts of this article, you’ll understand how hard it was technically to get to this point. It seems a bit of a nightmare to make the whole system work properly all the time.

All of this is good and fine but back to the point. How to avoid problems that are caused by false readings? Namely, how to avoid a wild goose chase?

The question lies somewhere in between trusting the OBD system and using common sense, preferably with a pinch of previous experience and query.

Of course, consider the error code, but take a look around the car and inside the engine bay. See if there are any visible and obvious faults.

One of the best examples of this kind of problem is a loose gas tank cap.

It will easily cause a check engine light to come up and if you were to hook up the diagnostics afterward, you would probably get a faulty O2 sensor code.

Instead of tightening or replacing a cheap cap,  you’d replace an expensive O2 sensor for nothing.

If you want to read more about these kinds of problems, click here for a separate article.

To conclude: rely on the diagnostic system but always have a shadow of a doubt. Don’t always rely solely on electronics. Sometimes let some good logic and obvious problem-solving help out.


Written by: Sibin Spasojevic


Former car technician, life-long car and DIY enthusiast, author for