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Stop Paying to Fix Your Car Without Knowing What's Wrong


Glenn Stok applies his engineering background to solve technological problems and save money on car repairs. He shares his insight below.


Some people spend hundreds of dollars on unnecessary and questionable repairs when they see the check engine light is on.

I’ll show you how to know ahead of time what to expect. I use a low-cost diagnostic scanner that displays the OBD error codes saved in my car’s computer.

What are OBD Error Codes?

The OBD error codes are stored in the car’s computer, known as the Powertrain Control Module (PCM). When a condition occurs that indicates a problem with emissions control, it is reported to the PCM via one of many sensors.

Sensors pass vital information to the PCM about the condition of the parts that they monitor. In some cases, the PCM analyzes the data to determine if the parts are functioning properly. In other cases, it’s just an indicator of a problem that needs attention.

What To Do When You Have an OBD Error Code

If you trust your mechanic, you can simply let them check your car and suggest what repairs are needed. But you’d still be left in the dark without knowing for sure what made the check engine light come on.

An OBD diagnostic scanner will tell you what code is stored in the PCM. Once you know the code, you can do a Google search to find more details about the situation.

If you have a failed unit, whatever it may be, it does need maintenance. Clearing the codes doesn’t make the problem go away. However, in situations where a sensor malfunctions and gives an erroneous fault code, only the sensor needs to be replaced. Sometimes you can determine this by clearing the code from the PCM with a diagnostic scanner.

In some cases, the problem could be intermittent with a sensor going bad. In that case, it stops sending the erroneous fault code. If you clear the code yourself and it never comes back on, then you’ll know it was just an intermittent problem. That happened to me, and I never had to have anything repaired.

Sometimes the computer eventually clears the code on its own, and the check engine light goes off. But it may retain a history, which will cause your next state emissions test to fail. So even if the light turned off, scanning the PCM with a diagnostic scanner will assure you that you’re good to go before bringing your car in for a state inspection.

Can the Powertrain Control Module Fail?

Everything can go wrong when the main computer, which is the powertrain control module (PCM), fails. That is reported with code P0601 when it fails a self-test. If you clear that code, it will quickly reappear as soon as the PCM reports a failing condition again.

Other codes also refer to problems with the PCM. For example, if the PCM malfunctions with controlling the air/fuel ratio, you’ll get an error code P2099.

Fortunately, it may not always be the PCM that’s bad. This situation can also be due to bad wiring or grounds on sensors that may have become rusty. Check those things first. If the PCM has failed, you’ll need to have it replaced. But that condition is rare.

If you do end up getting a new PCM, I recommend getting the original manufacturers equipment. After-market PCMs can be faulty.

Is a Repair Always Necessary When the Check Engine Light Is On?

As I mentioned earlier, some error conditions are intermittent and usually due to a malfunctioning sensor.

If the light keeps coming back on after you clear it, you should have a mechanic review the status and determine what further steps you need to take.

Some mechanics will try the cheaper solution first, by replacing the sensor. Or they will check the wiring, which could also cause an erroneous error code. The check engine light should not come back on after a faulty sensor or bad wiring has been replaced—if that’s all it was.

What if the Check Engine Light Comes Back On After the Repair?

A reoccurring OBD code, after a repair, might mean that it’s not the replaced part that was bad. If you had an expensive part replaced, ask the mechanic if they tested the sensors or if they examined the vehicle for other causes of the error condition.

If you had only the sensors replaced, the part that the OBD code refers to is most likely malfunctioning and needs to be replaced.

However, a good mechanic will also check the wiring to the part in question. Damaged wires can also contribute to a misleading error condition.

Beware of Dishonest Auto Repair Shops

If you have a failed unit, it does need maintenance. Clearing the codes doesn’t make the problem go away. However, when a sensor malfunctions, only the sensor needs to be replaced.

Some repair shops will not check the sensor. They'll just sell you the expensive part that may not be malfunctioning. They should rule out a sensor acting erratically.

A dishonest mechanic might insist you need a new transmission if you have code P0700. However, it does not necessarily mean you need to replace it. That code only indicates a fault in the transmission. A good mechanic will get a reading from the transmission module to determine the exact problem.

Another common code is P0420, which indicates a problem with the catalytic converter. That’s one of the most expensive items to replace, and some cars have two. Minor issues can cause an erroneous P0420 code, and I’ll discuss that next.

Understanding the Catalytic Converter OBD Code P0420

The catalytic converter burns up hard particle pollution and unburnt fuel in the exhaust system. But it only can do this after it heats up and it can only handle a certain amount of unburnt fuel.

Some cars have two catalytic converters. One on each side of the engine, known as bank 1 and bank 2. These function better because they heat up faster due to being close to the engine.

There is an oxygen sensor before and after each catalytic converter. The reason why two sensors are needed is to report to the PCM if the proper air/fuel ratio is not being achieved. The PCM compares the readings taken before and after the catalytic converter to determine if it’s functioning efficiently.

If too much unburnt fuel gets into the catalytic converter, it won’t be able to burn all that up, and it will result in a P0420 code (or P0430 for bank 2).

That can also happen if the gasoline you use has water in it. Before spending money on a new catalytic converter, try eliminating that issue by using a different gas from another gas station. The error code might clear after a while on its own.

Better yet, drive with a tank full of high octane gas. That might clear things up since some high-octane fuels contain more detergents. That worked for me. I got ten more years out of my car without having to buy a new catalytic converter after the first time the check engine light indicated that there was a problem.

Be Cautious of After-Market Modifications

Cheap replacement parts can cause problems that affect the function of the catalytic converter.

You should use only original equipment spark plugs and wires when you have them replaced. Other brands may cause poor combustion, which can contribute to a buildup of unburnt fuel deposits in the catalytic converter.

Does the Light Clear if the Battery Is Disconnected?

When you disconnect the battery, the PCM will lose the data, and your check engine light will be off when you reconnect it. But the PCM will have a “not ready” status that clears after driving up to 100 miles in several sessions.

If you plan to get your state's emissions inspection, you will not pass until a “ready” status appears. The diagnostic scanner will show you the status.

How to Tell Why Check-Engine Light Is On With a Diagnostic Scanner

In some cases, a sensor will malfunction and return an erroneous code. A trustworthy mechanic can determine if the problem is due to the actual part that the error code refers to, or caused by a bad sensor.

The low-cost diagnostic scanner that I use may not reveal all stored codes. But it will give you a heads-up as to what’s going on with your vehicle. It also can be used to clear the codes, as I discussed in this article. That’s helpful to eliminate intermittent problems that sometimes clear up on their own.

Automobile repair shops have more sophisticated scan tools that provide freeze-frame data to help with the diagnosis of more involved problems.

Nevertheless, you need to know the facts about the condition so you know what to expect.

You can confirm the error codes with your own low-cost OBD scanner from Amazon, similar to the one that I use. This inexpensive device tells you why your "Check Engine Light" is on.


Common OBD Codes

The following are the most common error conditions that people have asked me about in comments in another article.

You should have your vehicle checked by a trusted auto mechanic to determine if repairs are required if you have any questionable error code. Remember, you can always do a Google search to find details on any code. Include the words “OBD Error” followed by the code in your search.

OBD CodeDescription


The variable valve timing (VVT) or variable camshaft timing (VCT) is off.

P0136, P0137

These codes refer to the downstream oxygen sensors, indicating that there is too much oxygen in the exhaust. This could mean the catalytic converter may not be functioning properly.


Bank one running too lean (Too much air or too little fuel).


Not all cylinders are firing properly and a random misfire is detected.


There is an erratic voltage from sensor B, which is the secondary crankshaft position (CKP) sensor. 


Indicates a timing problem with the camshaft position sensor. It could just be bad wiring from the sensor to the PCM.

P0420, P0430

Catalytic converter efficiency problem. Discussed above.


The Gas cap is loose or doesn’t close properly. Or the EVAP system is damaged that should be recovering fuel vapor.


Low System Voltage


The PCM computer failed a self test. Discussed above.


Indicates a fault with the Transmission Control System.


The PCM is malfunctioning. It’s failing to control fuel trims to keep the air/fuel ratio correct.

People Also Ask

My car already failed inspection because the check engine light was on. They told me it was the catalytic converter. Do I now have to get my car fixed?

That all depends on what is causing the problem. It might just be faulty oxygen sensors. It would be best if you had an honest mechanic check your car. They have more sophisticated diagnostic tools that help analyze more involved emissions problems.

Is there more than one sensor for the catalytic converter?

There are two oxygen sensors for each catalytic converter, one before and one after. The sensor before (upstream of) the catalytic converter detects the air/fuel ratio in the exhaust and the one after (downstream), detects the efficiency of the catalytic converter. Some vehicles have two converters, so in that case, you’d have four sensors.

I had error code P0420 and replaced both o2 sensors and one catalytic converter. But the light still keeps coming back on. What else could be wrong?

A good mechanic would check related issues. You might have bad connections or damaged wiring connected to the sensors. If you were always buying your gas from the same station, try switching brands. They might have water in the gas. That could interfere with the efficiency of the catalytic converter.

What’s the difference between error code P0420 and P0430? Don’t they refer to the same thing?

They both mean that the catalytic converter may be operating below the required threshold for required efficiency standards. P0420 refers to “Bank 1” and P0430 refers to “Bank 2.”

How do I know for sure that I need to replace the catalytic converter when the check engine light is on with OBD error code P0420?

You don’t know for sure. It could be something minor, like faulty wiring to the sensors. Or an oxygen sensor could be giving false voltage readings. You might even merely have water that got into your gas. It would help if you ruled these things out before spending money on an expensive catalytic converter replacement.


© 2020 Glenn Stok


Christopher Hundley from Pennsylvania on August 04, 2020:

This is really good and useful advice for anyone buying a used car, especially models notorious for high annual maintenance costs.

Glenn Stok (author) from Long Island, NY on January 16, 2020:

Brenda Arledge - That’s a very meaningful question you asked about when they started putting diagnostic codes in vehicles.

It began in 1966 when California initiated the requirement for exhaust monitoring of tailpipe emissions. Then the entire United States made it a standard in 1968 with the first On-Board Diagnostics (OBD1).

OBD1 was not a standard among all vehicles. Every manufacturer implemented it with different protocols for diagnostics. As a consequence, there was a unique OBD1 scanner for every brand of car.

OBD2 became a standard requirement in 1996 for all cars are light trucks. From then until today any OBD2 diagnostic scanner can be used for all brands of vehicles made since 1996, by connecting to its universal connector under the dash.

BRENDA ARLEDGE on January 16, 2020:


A lot of information in this article.

I remember having a lot of trouble with a vehicle I had in the past, and the mechanic kept showing me these codes.

I did not even know they existed. But I do remember that the solution was not fixed. Each time it would say one thing, and I would have it fixed, only to have the light come back on once more. I believe I had to replace the sensor.

When did they start putting these in vehicles? Is this something that was in our older cars without the bells and whistles?

Thanks for sharing. Interesting write.

Glenn Stok (author) from Long Island, NY on January 12, 2020:

Liz Westwood - Good question Liz. In many ways technology makes products more complex for the general user.

Liz Westwood from UK on January 12, 2020:

This is a useful article for car owners. It makes me wonder, though, where we better off before we drove cars loaded with technology?

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