Check Engine Light On? Why Reading the Code Isn't the Same as Diagnosing the Problem
Your check engine light comes on and instantly you feel a little anxiety or maybe confusion.
Maybe the vehicle seems to be running perfectly normally. Maybe it feels a little different. Or maybe something is obviously wrong. Someone plugs a scanner into the vehicle and retrieves a code. Maybe you have your own code reader. Maybe a friend scans it. Maybe you stop somewhere that offers code reading.
Now you have something that looks very specific: a letter, four numbers and a short description. So now you know what's wrong with the vehicle, right?
Not necessarily.
Reading a diagnostic trouble code is an important first step. But the code is information from the vehicle's computer. Diagnosing why that code appeared is a separate process.
That's an important distinction because a trouble code does not necessarily identify a failed part.
So, what does your check engine light actually mean? What information does a scan tool provide? Why can the same code have several possible causes? And what does a technician do after retrieving the code?
That's what we're going to explain.
No automotive background required.
What Does the Check Engine Light Mean?
The check engine light, also called the malfunction indicator lamp or MIL, is part of your vehicle's onboard diagnostic system.
Modern vehicles continuously monitor numerous systems and operating conditions. When the vehicle's computer detects certain conditions outside the expected range, it can store diagnostic information and illuminate the check engine light.
Depending on the vehicle and the problem detected, the light may appear while the vehicle seems to drive normally, or it may be accompanied by noticeable symptoms.
The important thing to understand is this:
The check engine light tells you the vehicle detected a problem. It does not tell you exactly what caused it.
That requires more information.
Why Can the Check Engine Light Come On for So Many Different Reasons?
Because the engine computer isn't monitoring just one component.
It receives information from sensors and monitors systems involving fuel delivery, ignition, emissions, airflow, engine operation and other electronically controlled functions. A problem in one area can sometimes affect the information being reported somewhere else.
That is one reason the light itself cannot tell you which repair the vehicle needs.
What Is a Diagnostic Trouble Code?
When certain monitored conditions meet the criteria for a fault, the vehicle can store a Diagnostic Trouble Code, usually shortened to DTC.
These are the letter-and-number combinations you may see after a vehicle is scanned.
For example:
P0171
That code has a specific meaning related to a condition the vehicle's computer detected.
But here's where an important misunderstanding happens.
A code description is not necessarily a diagnosis.
It tells the technician where the diagnostic process should begin.
Does a Trouble Code Tell You Which Part Is Bad?
Sometimes a code may lead fairly quickly toward a particular component or circuit.
Other times, several completely different problems can produce the same code.
That's why reading:
P0171
doesn't automatically translate to, replace this part.
A component named in a code description may be involved in the system being monitored without being the component that actually caused the problem.
Reading a Code vs. Diagnosing the Problem
This is the distinction we want vehicle owners to understand.
Scanning Gives You Information
Connecting a scan tool and retrieving stored codes gives the technician information about what the vehicle's computers have detected. Depending on the vehicle and scan equipment, additional information may also be available, including operating data and other stored information.
While that information is extremely useful, it's only the beginning of the process.
Testing Provides Evidence
Once the technician knows what the vehicle detected, the next question is:
Why did it detect it?
That may require inspecting components, reviewing data, checking electrical circuits, measuring pressures or voltages, looking for leaks, verifying mechanical operation or performing other tests appropriate to that particular vehicle and fault.
The exact testing depends on the code, vehicle, symptoms and system involved.
Diagnosis Identifies the Cause
The purpose of diagnosis is to take the information provided by the vehicle and determine the actual cause of the problem.
A simple way to remember it is:
Scan → Information
Testing → Evidence
Diagnosis → Cause
And only after the cause has been identified can the appropriate repair be determined.
Why Can One Trouble Code Have Several Possible Causes?
Because vehicle systems interact with one another. The computer makes decisions based on the information it receives and the conditions it observes. When something falls outside an expected range, the computer may identify the condition without knowing which physical problem caused it.
This is easier to understand with a real example.
A Real Example: P0171
P0171 is commonly associated with a system-too-lean condition on Bank 1.
It would be tempting to look at that description, identify a sensor associated with the fuel or exhaust system and replace it.
But look at what can actually be involved. In one GM service procedure for P0171/P0174, diagnosis includes reviewing long-term fuel trim and evaluating items such as MAP and MAF sensor data, vacuum hoses, fuel pressure, fuel contamination, injectors, exhaust leaks, intake leaks, EVAP components, crankcase ventilation, oxygen-sensor installation and circuits, and possible engine-related problems.
That's a much different picture than:
“The scanner told me which part to buy.”
The Code Describes What the Computer Detected
With a lean-condition code, the important first question isn't:
Which sensor should we replace?
It's: Why is the computer seeing this condition?
Possible causes can exist in air intake, fuel delivery, exhaust, electrical or other related systems depending on the vehicle. Different causes can produce the same result from the computer's perspective.
That's why the correct diagnosis matters and that is why no matter what code you tell us an autoparts store read for you, we still need to properly diagnose that trouble code.
What Does a Technician Do After Reading the Code?
The answer depends on the vehicle and the code.
There isn't one universal diagnostic procedure for every check engine light.
The technician may begin by reviewing all stored codes rather than focusing on only one. Related codes can provide additional clues about what's happening.
From there, diagnosis may involve a combination of vehicle data, visual inspection, manufacturer service information and physical testing.
What Is Freeze-Frame Data?
When some faults are recorded, the vehicle may also store information about operating conditions when the fault occurred.
This is commonly called freeze-frame data.
Depending on the vehicle, it may provide information such as engine speed, temperature, load or other operating parameters.
That can matter because some problems occur only under particular conditions.
Manufacturer diagnostic information can specifically direct technicians to use freeze-frame information to reproduce the conditions present when a fault occurred. Ford service information addressing lean-condition codes, for example, notes that freeze-frame data can help identify the engine and ambient conditions present when the DTC was set.
What Is Live Data?
A capable scan tool can also allow technicians to observe information being reported by the vehicle while it is operating.
Instead of simply asking: What code is stored?
The technician can begin asking: What is the vehicle doing right now?
Those data points can then be compared with expected values and other test results.
What Other Tests Might Be Needed?
Depending on the problem, diagnosis may involve things such as electrical testing, circuit checks, fuel-pressure testing, smoke or leak testing, visual inspection or other system-specific procedures.
The testing should follow the evidence. That's the difference between diagnosing a problem and simply replacing the first part that seems related to the code.
Why Isn't a Free Code Scan the Same as a Diagnosis?
A code scan can be useful.
If someone retrieves the stored DTC from your vehicle, you now have information you didn't have before. There is nothing wrong with that.
The problem occurs when code retrieval is presented as a complete diagnosis.
If a scanner retrieves P0171, the scanner has successfully told you that the vehicle stored P0171. It has not inspected the intake system, it has not measured fuel pressure, it has not tested a circuit, it has not found a vacuum leak, it has not proven that a particular sensor failed. Those are different steps.
So if you've already had your vehicle scanned before bringing it to a repair shop, tell the technician which code was retrieved. That information can be helpful, just don't be surprised when additional diagnostic testing is still necessary.
Why Not Just Replace the Part Mentioned by the Scanner?
Oh yeah, we could totally do that. But when the issue isn't fixed, you'll call us telling us how upset you are because we didn't fix your issue. What if the part isn't the problem and replacing a functioning component doesn't repair the condition that caused the code?
The vehicle may return with the same check engine light, the same DTC and the same underlying problem. And now you've paid for a part and labor without actually fixing the vehicle.
This is sometimes called parts swapping or firing the parts cannon: replacing components based on possibilities rather than evidence.
At Cherry Grove Automotive, our goal is different.
Find the cause first, and then repair the cause second.
Can Two Vehicles With the Same Trouble Code Need Different Repairs?
Absolutely.
This is one of the most important reasons a DTC shouldn't be treated as a universal repair instruction.
Two vehicles can store the same code but have different underlying problems.
Even manufacturer diagnostic procedures for P0171 demonstrate how vehicle-specific this process can be. A 2012 Toyota Sienna procedure, for example, includes checks involving airflow data, fuel pressure, exhaust leakage and ignition, while other manufacturers' procedures for the same basic code can follow different sequences and specifications.
The code may be the same, but the diagnosis may not be. And the repair certainly doesn't have to be.
Can I Diagnose a Check Engine Light Myself?
You can certainly retrieve codes yourself with an appropriate scan tool or code reader.
For someone interested in understanding their vehicle, that's useful information.
You may also be able to identify something simple. But whether you can completely diagnose the problem depends on the fault, the vehicle, the information available and the testing required.
Some problems require specialized equipment or access to vehicle-specific diagnostic information. Others require electrical, mechanical or system testing beyond what a basic code reader can provide.
So there's nothing wrong with scanning your own vehicle.
Just remember:
Reading the code and proving the cause are two different things.
Can You Drive With the Check Engine Light On?
There isn't one answer that applies to every check engine light.
The severity depends on what caused the light, how the vehicle is behaving and the manufacturer's guidance for that vehicle.
A check engine light can represent a wide range of faults, which is another reason the light itself shouldn't be used as a diagnosis. Manufacturer guidance commonly directs drivers to have a vehicle inspected when the malfunction indicator lamp illuminates.
What If the Vehicle Is Running Poorly?
If the check engine light appears along with significant shaking, loss of power, unusual noises, overheating, smoke or other serious symptoms, don't assume it's safe to continue driving simply because the vehicle still moves.
The safest response depends on the symptoms and the vehicle. If you're uncertain, have the vehicle inspected rather than continuing to drive and hoping the light goes away.
What About a Flashing Check Engine Light?
A flashing check engine light deserves immediate attention. Exactly what a flashing malfunction indicator means and the recommended response can vary by manufacturer and vehicle, so the owner's manual is the correct source for vehicle-specific guidance.
If the check engine light is flashing or the vehicle is running very poorly, reduce the risk of additional damage by getting the vehicle safely stopped and having the situation evaluated rather than continuing to drive normally.
Will Clearing the Code Fix the Check Engine Light?
Clearing a diagnostic trouble code can turn the warning light off temporarily.
That does not mean the underlying problem has been repaired. If the condition that caused the code is still present and the vehicle detects it again, the code and warning light may return.
Clearing codes can also erase diagnostic information that may be useful in finding an intermittent problem. So clearing the code is not the same thing as repairing the vehicle.
Why Did My Check Engine Light Turn Off by Itself?
Sometimes a check engine light can turn off after the vehicle no longer detects the condition for a required number of drive cycles.
That doesn't necessarily mean nothing happened. The vehicle may still retain diagnostic information, depending on the system and circumstances.
Intermittent problems are one reason information such as stored codes, fault history and freeze-frame data can be valuable.
If the light has been coming and going, tell the technician.
“It isn't on right now” doesn't necessarily mean “there isn't anything to diagnose.”
Check Engine Light Diagnosis in North Myrtle Beach
If your check engine light is on, the first step isn't automatically replacing a part.
It's determining why the vehicle turned the light on.
Cherry Grove Automotive diagnoses check engine light and drivability concerns for cars, trucks and SUVs in North Myrtle Beach.
We Diagnose Before We Replace
Diagnostic testing may involve retrieving stored trouble codes, reviewing vehicle data, inspecting related systems and performing the tests necessary to determine the cause.
If testing identifies a failed component, we can explain what was found and what repair is recommended.
And if the component that appeared to be the obvious answer isn't actually the problem, that's important information too.
Replacing a part because it might be the problem isn't the same thing as diagnosing the vehicle.
Check Engine Light and Diagnostic Service at Cherry Grove Automotive
If your check engine light is on, you can request an appointment online or call Cherry Grove Automotive at 843-249-6563.
Online appointment requests are not confirmed until you receive confirmation from Cherry Grove Automotive.
Cherry Grove Automotive 1012 Sea Mountain HighwayNorth Myrtle Beach, SC 29582
843-249-6563
Have a Question About Your Vehicle?
This article is part of our Understanding Your Vehicle series, where we explain vehicle parts, warning lights, systems and repairs in plain English.
Have a question about something your vehicle is doing? Let us know. It might become a future article.
You don't need to know everything about your vehicle, but we're excited to help you understand it.
