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How to Read Your Vehicle’s Check Engine Light Trouble Codes

As a technical writer with a science and engineering background, I solve technological problems to save money on car repairs.

ANCEL AD310 OBD-II Diagnostic Scanner that I use.

ANCEL AD310 OBD-II Diagnostic Scanner that I use.

Why I Bought a Code Scanner

I discovered how crucial it is to read my car's diagnostic codes and know what they mean before bringing my car in for repair. That always gives a clue of what’s wrong, and knowing that could help avoid costly car repairs.

For that reason, I found it beneficial to have my own diagnostic scanner to find out what’s wrong when the Check Engine Light is on. This article is a review of the ANCEL AD310 diagnostic scanner that I use.

What you’ll get from this article:

  1. First, a quick explanation of what the check engine light does.
  2. Know what to do if you have a problem.
  3. Understand if repairs are always necessary.
  4. And finally, I’ll give you a thorough review of the diagnostic scanner I use.

What Does the Check Engine Light Indicate?

The Check Engine Light (CEL), which is also known as the Malfunction Indicator Lamp (MIL), indicates that a specific sensor has reported a problem.

  • A code that specifies the problem is stored in your vehicle’s computer, the Powertrain Control Module (PCM). The PCM also controls many of your car’s functions.
  • The codes are known as On-Board-Diagnostic (OBD) trouble codes. A new standard for all vehicles and light-duty trucks began in 1996 as OBD-II.

Sensors pass vital information to the PCM about the condition of the parts that they monitor. The PCM analyzes the data to determine if the parts are functioning properly.

In some rare cases, the CEL will flash repeatedly. That indicates a problem that needs immediate attention, such as an engine misfire that can cause significant damage.

What to Do When You Have an OBD Error Code

The problem could be intermittent, with a sensor going bad. In that case, it could stop sending the erroneous fault code.

If you clear the code yourself and it never comes back on, you'll know it was just an intermittent problem. That happened to me, and I never had to have anything repaired.

Sometimes the code eventually clears 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 is turned off, checking it with a diagnostic scanner will assure you that you're good to go before bringing your car in for a state inspection.

Is a Repair Always Necessary When a Code is Found?

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

The OBD Diagnostic Scanner That I Use

You can spend hundreds on a diagnostic scanner that gives detailed information. However, I found this low-cost scanner for under $35 that does more than other scanners in that price range. In addition to reading the OBD trouble codes and resetting the Check Engine Light, it also displays information about the car’s performance.

I had an older scanner that still works (they never become obsolete), but I recently bought the ANCEL AD310 diagnostic scanner on Amazon that does much more. It’s the best I’ve found, and I decided to upgrade to it for several reasons:

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  1. It has a complete built-in code library, so it shows the description of the error codes on the display that's easy to understand.
  2. You can view live data in real-time, such as the engine’s RPM, vehicle speed, temperature, spark events, and more. In my opinion, I find it worthwhile to monitor the car’s performance. An example of the display is in the image farther below under “Real-Time Live Data Stream.”
  3. In addition, it shows the data that service techs like to see. For example, you can view system information such as open-loop, closed-loop, and fuel system status. That may not be as interesting, but it’s great that this tool includes that ability for those who can make use of it.

What Vehicles Are Supported?

The ANCEL AD310 scanner supports all current OBD-II protocols.

OBD-II is used in all vehicles sold in the United States since 1996, Canada since 1998, and gasoline vehicles sold in the European Union since 2001.

How Easy Is It to Use?

Just connect the ANCEL AD310 to the OBD-II Data Port that’s under the dash on the driver’s side in most vehicles. It uses power from the data port, so no batteries are needed.

The cable is only 2 ½ feet long, but I found that to be a benefit. My older scanner had a long cord that got tangled often. This one is just the right length since you do all your monitoring while you’re in the driver’s seat anyway.

When you plug it in, it will tell you to set the ignition on (that is, without the engine running) and then press any key to connect. It then takes a few seconds to link with the vehicle, as shown here:

ANCEL AD310 Linking to vehicle when first started.

ANCEL AD310 Linking to vehicle when first started.

Four Functions are Possible

There are four functions it can perform, and it clearly prompts you through its use. I didn't even need to read the user's manual (Although I did anyway).

  1. Read and clear trouble codes.
  2. Check I/M readiness status to know if your car is ready for the state emissions test.
  3. View Freeze Frame data recorded at time of a fault.
  4. Monitor real-time live data.

For the first three functions, turn on ignition without starting the engine. To monitor real-time data, you’ll need to start the car.

You’ll probably use it typically to find out what trouble codes are stored when the check engine light is on. And you can use it to clear the codes (that turns off the CEL). The purpose of doing that is to see if the light comes back on. Sometimes it’s due to a faulty sensor that gives intermittent erroneous warnings. In that case, the light would stay off after resetting it.

When you do find a reoccurring problem, you’ll need to bring your car to a mechanic. But at least you’ll know what to expect and not be taken by a dishonest repair shop.

ANCEL AD310 Diagnostic Menu

ANCEL AD310 Diagnostic Menu

Read Codes With the Built-in Code Library

Other scanners only show the code, and you have to look them up online to know what they mean. But the ANCEL AD310 displays the description along with the trouble code, as shown with the example in the image below.

Example showing diagnostic code displayed.

Example showing diagnostic code displayed.

Freeze Frame Data Display

Automobile repair shops have sophisticated scanner that provide freeze-frame data to help diagnose more complex problems by analyzing the data recorded at the time of a fault.

I like the fact that the ANCEL AD310 shows this information. Freeze-frame data is a snapshot of the vehicle’s operating conditions when an emission-related fault occurred.

Real-Time Live Data Stream

I find it worthwhile to check the real-time data stream, especially with older cars. This data is shown while the engine is running.

The example in the image below is only one of several screens of data. You can press the up and down arrows to display other items. The data includes RPM, vehicle speed, voltage, temperature, fuel system status, spark events, and more.

ANCEL AD310 Showing Real-Time Data

ANCEL AD310 Showing Real-Time Data

I/M Readiness Status

After you have any repair done that was necessary due to a reported OBD code, various parts may not be immediately ready for a state emissions inspection.

Also, if you disconnect the battery for any particular reason, the PCM will lose the data about any existing trouble codes. So your CEL will be off when you reconnect it, but the PCM will have a “not ready” status.

That is also the case if you clear the codes with the scanner. You will not pass your state’s emissions inspection until a “ready” status appears for all the related monitors.

The ready status clears after driving up to 100 miles in several sessions as long as no faults are detected that would place new codes in the PCM.

The ANCEL AD310 displays the status of all emissions-critical items necessary to be ready for an emissions test, as shown in the image below. You need to press the down arrow button to scroll through all the items to verify that they are ready.

ANCEL AD310 Showing I/M Readiness status of the first three items. Press down arrow for more.

ANCEL AD310 Showing I/M Readiness status of the first three items. Press down arrow for more.

Two Most Common Reasons for the Check Engine Light

  1. It’s common for people not to tighten the gas cap properly. That will give OBD code P0455 or P0442. Once you tighten the gas cap, the system will detect proper pressure after driving several miles, and the code will clear automatically.
  2. Another common issue is with the catalytic converter, showing OBD code P0420. That’s one of the most expensive items to replace, but a faulty sensor or other minor issues can cause an erroneous P0420 code.

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.

Mistakes in judgment and failure to follow through with a complete diagnosis can produce unneeded repairs that don't fix the problem.

So make sure you know of an honest repair shop that you trust. The use of an OBD diagnostic scanner gives you peace of mind but doesn't replace due diligence.

Final Thoughts

The ability to read the OBD diagnostic codes and monitor your car's performance helps you to be better informed about its operating condition before bringing it in for repair. That will help you save money by avoiding unnecessary work on your vehicle. The ANCEL AD310 OBD Scanner has spared me grief many times already.


© 2020 Glenn Stok

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