How to Find Out Why "Check Engine Light" Is On and Avoid Costly Repairs
If your "check engine light" is on and your mechanic says you need some expensive repair, there is an easy way for you to check whether he is telling the truth. You can read your car's diagnostic trouble codes yourself with a low-cost Diagnostic Code Scanner that plugs in under the dash to access the car’s computer.
This article will show you how to understand what your check engine light means, whether there is a real problem or just a faulty sensor, how to reset the light, and potentially avoid expensive car repair bills.
There are a number of component failures that turn on the check engine light. One of the most expensive to repair is the catalytic converter. However, many times the light is triggered by nothing more than a faulty sensor.
Here's what to expect from this article:
- I'm going to explain how to read your car’s diagnostic codes.
- I will also explain how to tell whether there is a real issue with your engine, or simply a faulty sensor that triggered the light.
- I'll show you how to clear the light so that you can pass an inspection.
- Some cars also have a "Check Tire Pressure" light. I'll discuss that near the end.
- Finally, I conclude with a section listing common questions readers have been asking, along with my answers.
Understanding Your Car's Diagnostic Codes
Your car's computer keeps a record of the diagnostic codes that represent which sensor is reporting a problem. The check engine light comes on when any sensor reports a faulty device. However, it may not be the device that is bad, it may just be an inexpensive faulty sensor.
When sensors fail, it is usually intermittent. A useful experiment is to clear the codes from the computer and watch to see if the check-engine light comes back on. Your car’s diagnostic codes can be read with a simple code scanner. It plugs into a special plug that you can find under the dash.
These scanners can also be used to reset your car’s computer by clearing any saved trouble codes.
Since I have a background in computer science, I am aware of an important issue: The only way the car's computer can know if a unit is failing is by a sensor reporting back to the computer. Each sensor specifies a particular code. When the computer has a code stored, the check engine light is lit to let you know. The computer registers a specific trouble code to indicate which sensor reported the problem. These codes are known as the OBD I and OBD II codes, but more on that in a moment.
Sensors Can Fail or Erroneously Trip
Your car has sensors for many components. Most are related to emissions control (see below for more about the tire pressure warning light on newer cars). Sometimes a sensor will fail or get stuck and report a bad code. An honest mechanic will tell you that. Replacing a sensor is not that expensive.
You can do your own cheap engine diagnostics by using one of these low-cost code scanners. This will help you discover why your check engine light is on. But knowing if it's a sensor problem takes some extra work, as I'll explain.
Choosing a Diagnostic Code Scanner
Many times I have noticed the mechanic plugging in a code scanner under the dash to read the engine trouble codes from the car's computer. I thought how nice it would be to know what the computer is saying about the check engine light before I go into the repair shop. So I decided to spend a little money on my very own Diagnostic Code Scanner.
I did a few searches for these units and found really good prices and a large choice of models on Amazon. Prices range from around $50 to the upper $200's depending on the model and features. After a little review, I realized that I didn't need to buy the most expensive one.
If you have a 1996 or newer car, then the cheaper models of Diagnostic Code Scanners will do just fine. All cars since 1996 use the OBD II codes. The more expensive Code Scanners can read the older OBD I codes as well. That is simply not necessary to have, unless you have a really old car.
Why It's Worth Getting an OBD Code Scanner
Replacing the catalytic converter can be expensive for parts and labor combined. If you are scheduled for your annual State Inspection then you will be forced to pay for the repair in order to pass the inspection.
The catalytic converter is part of the automobile exhaust system, placed between the engine and the muffler. It reduces the toxicity of emissions from internal combustion engines.
It was first introduced in the U.S. in the mid-1970s to comply with EPA regulations for controlling auto exhaust. Its functions are monitored today by the car's computer system with sensors that are on all important auto parts.
The sensor that detects a problem with the catalytic converter is known to erroneously trip sometimes. It happened to me and to several friends. I never needed a replacement because it was just a tripped sensor causing the check engine light to come on.
The catalytic converter typically has a warranty exceeding the rest of the automobile's warranty length. However, if you are beyond that warranty period (by time or by mileage) then you could be facing a large repair bill.
If you're in a state that implements vehicle emission inspection programs, you might also get stuck with needing it repaired quickly because your car failed.
You may be able to avoid replacing catalytic converter. In many cases it's just a bad sensor and you wouldn't know that unless you have the ability to clear the car's diagnostic codes and watch to see if the check engine light comes back on.
The Diagnostic Scanner I Use
I originally purchased an older version Diagnostic Code Scanner, but there are newer ones available now in auto parts stores and on Amazon, such as this one. This thing works great! It shows On-Screen Definitions for OBDII Vehicles just like the one I use. You can flip through a menu of options on the display screen to read the computer for any pending problem codes, read the status of the individual sensors, and even determine if the check engine light is on when it should be off, or visa versa.
This scanner doesn't require me to look up the codes in a table either. It shows the description of the codes on the screen. It also lets you reset the check engine light by clearing the codes from the computer.
Most diagnostic scanners offer similar features. Actron has versions that also read OBD I codes in older vehicles with an optional cable. By reading the codes myself, I was able to see that my problem was indeed the catalytic converter.
This Is a Useful Video About Reading Trouble Codes
How to Read Your Code Scanner
Each code scanner is different, but they all show you the common problem codes and have a function that allows clearing and resetting the computer. I recommend that you read the user's manual. Most good scanners will guide you through the process on the little screen if you carefully follow along with the on-screen prompts.
How to Tell if There Really Is an Engine Problem
There is no way to know right away if the light is on due to a faulty sensor. You need to erase the code and then see if it comes back on over time.
My scanner gave me all the features I needed to read the codes and erase them. I first chose the option to scan for any trouble codes in the computer. My scanner also shows the meaning of the code in simple English. It's good to know this before erasing the codes. Then I chose the option to erase the codes from the computer.
But you're not done yet! In order to know for sure whether the issue is a faulty sensor or a catalytic converter that needs repair, you need to follow a few more steps.
When you clear the codes with the "Erase Codes" function, the status of the System Monitors is set to “Not Ready.” You have to drive 50 to 100 miles in several individual trips until the system reads the status of all the components again. "Several trips" means that you shut off the engine and start another trip. So it's not just 100 miles in one trip. It's more like doing three trips of about 30 miles each.
Then connect the scanner again and read the System Status to see if the "Not Ready" condition is gone.
This applies if you were to take your car in for an inspection. Simple answer, you will not pass inspection if the light is not off. This is accomplished by clearing the codes from the computer, but in order to pass inspection, the computer has to show a "ready status." If you were to bring your car in for an inspection the mechanic would tell you that he or she can't pass the car at that time.
Diagnosing the Problem
After you reset the codes, either of two things will happen.
- If the "Not Ready" status becomes "Ready" and the check engine light did not come back on, then the problem may have been an intermittent faulty sensor.
- If the check engine light does come back on then the scanner will show you that bad code again. That would indicate you do indeed have a component failure as specified by the code. In that case at least you'll know you will be spending money to fix a real problem.
In case you're wondering whether you blew the light by turning it off, that is definitely not the case. First of all, the Diagnostic Code Scanner checks the condition of the check engine light as well. Secondly, when you turn on the ignition without starting the engine, all the panel lights light up temporarily as a test so you can see they all are working.
Three System Status Conditions You Need to Understand
If the check engine light did not come back on, the system is working properly and your car will pass emissions tests.
Your car has not been driven enough after resetting the codes. You need to drive until the system indicates a READY condition.
This means that your car does not support that status monitor and you don't need to be concerned about it.
Do you have a question?
See "Questions From Readers" in the last section below, along with the answers.
High-End Code Scanner with More Features
Some people prefer to spend more money in order to have many more features that replicate what can be done by service mechanics. I don't find any need to this just to monitor our own issues when the check engine light comes one. With a simple scanner like the ones I mentioned above, you can get a good idea of the problem, and then bring your car in for a more precise checkup by a mechanic you trust.
Nevertheless, if you want a scanner that does a lot more, the INNOVA 3160 may be for you.
Tire Pressure Monitoring System (TPMS)
Understanding the Check Tire Pressure Light
In 2008 the National Highway Traffic Safety Administration introduced a new requirement that all new cars and light trucks must have a Tire Pressure Monitoring System, known as TPMS.
These newer cars and trucks have a TPMS warning light in the dash, which will come on if the pressure on any of the tires is bellow 25% of the recommended pressure. A flashing or blinking TPMS light means that one of the TPMS sensors is malfunctioning.
The TPMS codes are not included in the ODB I or II codes and will not be recognized by an OBD scanner. A company named Accutire makes a tire pressure monitor specifically for the purpose of reading the TPMS codes.
This scanner displays diagnostics results of Sensor ID, Tire Pressure, Temperature, Battery Condition and OE part number. Unlike the method of reading the OBD codes, no physical connection is required to read the status of the TPMS. It reads the data via wireless signals.
There are two methods for the TPMS system to work. One method is indirect using the ABS/wheel-speed sensors (wheels spin at different speeds based on their inflation). The other method is a direct valve sensor.
If your “Tire Pressure Light” is on, check the pressure in each tire. The recommended inflation pressure for most passenger car tires is 32 to 34 PSI when cold. But check your car's owner's manual for its proper pressure. Drive a while after properly inflating your tires to give the system a chance to register that the pressure is okay.
If the TPMS light does not go off after driving a while with the correct inflation, or if it's flashing, it could mean a fault with the tire pressure sensor, or with some other part of the monitoring system. You will need to find out what error code you have.
Some after-market wheels are known to have an issue. The sensor will not fit properly if the valve hole is in the middle of the wheel’s barrel. If you are buying new wheels, check with your dealer to see if they have wheels that are TPMS-compatible.
Questions From Readers About the Check Engine Light
These are common questions people have been asking, along with my answers.
- Does the check engine light eventually turn off without fixing the problem?
If the problem is not due to an actual component, but rather just a faulty sensor which is intermittent, then you may notice the light goes off after some time. But it may come back on again once is a while if the sensor continues to send faulty signals. It's best to have a mechanic look at it, but tell them that it seems to be a faulty sensor so they know you already have some knowledge of the situation.
I know two friends who paid dearly for a new expensive part, only to have the light come back on weeks later. The mechanic refused to refund the cost, stating that he had no way of knowing it was only a bad sensor. Don't fall for that.
- Is the catalytic converter connected to the car's computer?
Yes, that's one of the parts that is monitored by the computer. If the Catalyst System Efficiency is below threshold then It registers error code P0420.
- Can the Actron scanner read individual sensors?
Yes, it will read all sensors and display the results. Then it let's you optionally clear the codes from the computer to start fresh.
- How many miles need to be driven to reset check engine light for inspection?
After you clear the computer you need to drive up to 100 miles to give it a chance to monitor all the sensors and register the results. Sometimes it may be complete in about 50 miles. You'll know when you connect the scanner and check the status. It tells you if ready or not ready. Wait until you see everything ready before going for an inspection.
- If you clear check engine codes with a scanner will it allow you to pass inspection?
If the problem is a faulty part, chances are good that the computer will read a new error and register a code again by the time you get into the shop for the inspection. The purpose of this article was not to cheat the system but to avoid costly repairs when only a sensor is at fault.
If it's only an intermittent problem with a sensor, it may take a while for the sensor to get stuck again and you will pass inspection. Unless the sensor is so bad that it registers a code again before you get your car inspected. In any case, remember to wait until the computer registers "ready" as I mentioned in the last answer.
- If I reset my light will it show up in state inspection?
As long as you wait for the computer to show a "ready status" then there will be no clue that you reset the computer. See my prior answer about how many miles to wait.
- I got a new catalytic converter and my check engine light is still coming back on. Why?
When the check engine light comes on and the scanner code shows that the catalytic converter is the cause, it could simply be the sensor that failed. That's why I recommend using your own scanner to clear the computer and see if the light comes back on. If the sensor is faulty, it may be intermittent.
When a car mechanic reads the code, they'll most likely sell you an expensive catalytic converter when all you needed was a new sensor. There is no way for them to know without clearing the computer and waiting to see what happens.
- Is it possible that it could be a loose gas cap?
A loose gas cap causes a fuel vapor leak sensor to register code number P0455 and the check engine light will come on. But this code may be indicating a more serious problem. You could have damage anywhere in the EVAP system, which captures and returns the fuel vapor. So, by all means, make sure your gas cap is tight. And if it's seal looks damaged, get a new gas cap. But if the light continues to come on and your scanner shows P0455, then get it checked out.
QUIZ: Test Your Knowledge of the Check Engine Lightview quiz statistics
Questions & Answers
- Helpful 11
How do you avoid a costly car repair by resetting the fault codes? The codes have been stored in the PCM (Powertrain Control Module) because of a sensor or valve that has either failed or has been acting erratically. So I would think some type of repair must be done. Resetting the codes doesn't make the problem go away. Am I missing something?
If you have a failed unit, whatever it may be, it does need maintenance. It’s true that clearing the codes doesn’t make the problem go away. However, in situations where a sensor malfunctions, giving an erroneous fault code, only the sensor needs to be replaced.
In some cases, it's only an intermittent problem with a sensor. In that case, it stops sending the erroneous fault code. Eventually, the computer clears the code on its own, and the check engine light goes off. I can say this for sure since it happened to me.
If you clear the code from the computer with an OBD Diagnostic Code Scanner and it was only an intermittent problem, the code will not necessarily reappear. If it does reappear, then it probably is a malfunctioning part, and it does need repair in that case.
Some repair shops will not check to see if it’s the sensor. They'll just sell you the expensive replacement of a part that may not actually be malfunctioning.
One more thing: There are fault codes that refer to the PCM itself, which is the car's computer. If the PCM has failed, it must be replaced. If you clear that code, it will quickly reappear as soon as the PCM reports a failing condition again.Helpful 7
Will the check engine light go off automatically if you change the faulty sensor?
The OBD diagnostic system will reset automatically after a while if the status clears up and is no longer detected. This may take up to over 100 miles of driving.
If you have it replaced by a mechanic, they will usually reset the status for you. You can always do this yourself with a low-cost diagnostic tool such as the one I mentioned in my article.
If it is only the sensor that is faulty, the check engine light should not come back on. If it was not shut off by the mechanic, it should eventually reset once the OBD system no longer detects a problem.Helpful 6
I'm getting code P0340 on my 2002 Oldsmobile Alero 2.2L, which indicates a camshaft position, sensor circuit malfunction. This engine doesn’t have a camshaft position sensor. I changed the crankshaft position sensor. I started the car before clearing the code. Will it still show as bad in the PCM? What else should I do to fix this code so I can pass emissions?
In addition to indicating a problem with the Camshaft Position Sensor, the P0340 code could also be indicating a problem with the wiring from the sensor to the PCM. It could also be a bad PCM as well.
The sensor reads the data off the camshaft gears, so the P0340 code could also mean a timing problem. Therefore, you should check the timing belt, it can be worn or lose.Helpful 6
What do the OBD codes P0300 and P0339 mean?
You can do any search yourself by putting the code into Google along with the keyword “OBD code.” The two you asked about are as follows:
• The P0300 OBD code indicates “Random Misfire Detected” - This means not all cylinders are firing properly.
• The P0339 OBD code indicates “Crankshaft Position Sensor B Circuit Intermittent” - It means that the powertrain control module (PCM) is detecting an erratic voltage from sensor B, which is the secondary crankshaft position (CKP) sensor.
In both cases, you need to have your mechanic check your vehicle to determine if repairs are required.Helpful 2
© 2009 Glenn Stok