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When dealing with your car’s diagnostic system, knowing whether the codes have been cleared recently can be very important.
Whether you’re a car owner trying to solve a problem or someone looking to buy a used car, this guide will help you figure out if someone has cleared those pesky Diagnostic Trouble Codes (DTCs) recently.
This could mean they’re trying to hide an issue, or maybe they’ve just reset the system after a repair.
Why It’s Important to Know If Codes Were Cleared
Let’s say you’re checking out a used car, and everything seems fine on the surface.
No warning lights, no strange noises—but wait. What if the previous owner just cleared all the codes?
Those cleared codes could have been hiding some serious problems, and now you wouldn’t know until something breaks down.
Related Reading: How to Clear Codes on Cars with Scanners
What You’ll Need to Check the Codes
To check if the codes have been cleared, you’ll need OBD2 scanner.
These scanners connect to your car’s onboard diagnostic system and let you see if any codes are still present, if any codes were cleared, and even if the car’s systems have been reset recently.
Some Popular Automotive Scanner:
ANCEL V6 Pro ( Most Recomended)
BlueDriver Bluetooth Pro OBD-II Scanner
Innova 6100P OBD2 Scanner
Autel MaxiCOM MK808
These tools range from basic models that are easy to use for anyone to more advanced ones that give you detailed information.
Make sure your scanner can read freeze frame data and readiness monitors, as these features are key to checking for recently cleared codes.
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How to Check If Codes Have Been Cleared
Step 1: Connect Your Car Diagnostic Scanner
The first step in verifying whether diagnostic codes have been recently cleared is to connect your car diagnostic scanner to your vehicle.
The OBD2 port is designed for easy access and is typically located under the dashboard, near the driver's seat.
This port allows the scanner to interface directly with the vehicle's Engine Control Unit (ECU), the central brain that monitors and controls various systems.
When connecting your automotive diagnostic scanner, ensure that the vehicle is in the correct mode for the scanner to communicate with the ECU.
Generally, this involves turning the ignition key to the "ON" position without starting the engine.
In this mode, the ECU is active, and the scanner can retrieve diagnostic data.
It's important to note that some vehicles may require the engine to be running for certain diagnostic functions, but for most checks, including code retrieval and readiness monitor status, having the ignition on is sufficient.
Once connected, the scanner will automatically power on (if it’s connected directly to the OBD-II port) and begin communicating with the vehicle's systems.
Step 2: Access Diagnostic Information
After successfully connecting your scanner for car, the next step is to access the vehicle’s diagnostic information.
Most scanners come with a user-friendly interface that allows you to navigate through various menus. To determine if codes have been cleared recently, focus on the Diagnostic Trouble Codes (DTCs) section.
This section displays both active and pending codes.
Active codes are current issues that the ECU has detected, while pending codes are potential issues that have not yet triggered a full alert.
If the DTC section shows no codes, this could indicate that the system was recently cleared.
However, a lack of codes isn't always definitive proof that the system was tampered with; it could also mean that the vehicle is genuinely issue-free at that moment.
It's important to look at the number of codes and their nature.
If the DTC section is entirely empty, and you're aware that the vehicle recently had issues, this might suggest that the codes were manually cleared.
Some advanced car scanner diagnostic allow you to see when the last code was cleared or provide a history of recent diagnostic activities, which can give further clues about any recent code clearance.
Step 3: Check Freeze Frame Data
Freeze frame data is an invaluable tool in vehicle diagnostics.
When a DTC is triggered, the ECU captures a snapshot of the vehicle's operating conditions at that precise moment.
This data includes crucial information such as the engine speed, throttle position, vehicle speed, engine load, coolant temperature, and more. Reviewing this data can provide significant insights into what was happening when the fault occurred.
If the freeze frame data is missing, incomplete, or shows a time stamp that suggests recent activity, it's a strong indicator that the codes were cleared to hide an issue.
Typically, the freeze frame data should correlate with the DTCs displayed in the scanner.
If you notice discrepancies, such as data that seems too recent or doesn’t match the vehicle's operational history, it’s a red flag that someone might have cleared the codes to reset the system.
Additionally, advanced car scan tools allow you to analyze multiple freeze frames if more than one code has been triggered.
Step 4: Review Readiness Monitors
Readiness Monitors play a critical role in vehicle diagnostics by running self-tests on various emission control systems.
When a DTC is cleared, either manually or after a repair, these monitors reset
and must complete a series of diagnostic self-tests to determine whether the vehicle's systems are functioning properly.
The process of resetting these monitors typically requires a specific number of driving cycles, which vary depending on the vehicle and the specific monitor.
Monitors related to the catalytic converter, oxygen sensors, and evaporative emissions system might need different driving conditions to complete their tests.
When you check the readiness monitors using your OBD2 scanner, you’ll usually find this information under a section labeled I/M Readiness or something similar. The scanner will indicate whether each monitor is Complete, Incomplete, or Not Supported.
Complete means that the monitor has successfully passed its self-tests, while Incomplete indicates that the monitor has not yet finished its tests since the last reset or code clearing.
If you observe that multiple readiness monitors are incomplete, especially after the vehicle has been driven a reasonable distance, this strongly suggests that the codes were recently cleared.
This is because these monitors would typically be complete under normal driving conditions.
An incomplete status means that the vehicle has not yet gone through enough drive cycles to perform all necessary tests, likely because the codes were cleared recently.
Step 5: Examine Permanent Codes
Permanent codes, also known as Permanent DTCs, are a safeguard implemented in the OBD-II system to prevent tampering and ensure that emissions-related issues are properly addressed.
Unlike regular DTCs that can be cleared manually with car diagnostic tool, permanent codes cannot be erased by any manual means.
They are stored in the ECU until the vehicle's onboard diagnostic system confirms that the fault has been corrected and that the affected system is functioning correctly over a number of drive cycles.
To check for permanent codes, navigate to the appropriate section in your scanner for cars, which may be labeled as Permanent DTCs or similar.
If any permanent codes are present, they indicate that the vehicle has experienced issues that have not been fully resolved, even if regular DTCs have been cleared.
The presence of these codes suggests that someone may have attempted to clear regular codes to hide issues, but the underlying problems remain unresolved.
Permanent codes are particularly valuable when buying a used car, as they provide a more accurate representation of the vehicle’s health, regardless of any recent code-clearing attempts.
If your scanner detects permanent codes, it's advisable to investigate further and ensure the necessary repairs have been made before considering the vehicle for purchase.
Step 6: Investigate the Code History
The code history is a powerful feature found in more advanced car diagnostic scanners, providing a detailed log of past diagnostic trouble codes and any related events.
This historical data can reveal patterns, such as recurring issues that were cleared multiple times without proper resolution, or a sudden absence of codes that could indicate recent tampering.
To access the code history, use your automotive Scanner's menu to navigate to the appropriate section, which may be labeled as DTC History, Diagnostic History, or similar.
Here, you can review a list of previously detected codes, including those that were cleared.
The car diagnostic tool may also display additional information, such as the frequency of each code, the specific conditions under which they were triggered, and whether they were manually cleared.
Analyzing this history allows you to determine whether a vehicle has been experiencing ongoing issues or if there have been recent attempts to reset the system.
If you notice that the same code appears multiple times in the history but is currently absent from the active DTCs, it could indicate that the code was cleared in an attempt to hide a persistent issue.
By carefully reviewing the code history, you can gain a more comprehensive understanding of the vehicle's diagnostic record and identify any potential red flags that might not be immediately obvious through a standard DTC scan.
Extra Tips
Use a High-Quality Scanner
Basic OBD-II scanners are great for simple checks, but if you need more detailed info, consider using a more advanced scanner.
These can give you manufacturer-specific codes and more in-depth diagnostics.
If you’re unsure about the results, it might be worth taking the car to a professional mechanic with a high-quality scanner.
Pay Attention to the Symptoms
If your car is showing problems—like rough idling, poor fuel economy, or an intermittent warning light—but the scanner shows no codes, there’s a good chance the codes were cleared recently.
Don’t ignore these symptoms just because the scanner says everything’s fine.
Be Careful When Buying a Used Car
When buying a used car, always check the readiness monitors and freeze frame data.
If these look unusual, it’s possible the seller cleared the codes to hide a problem.
It’s always a good idea to have a professional mechanic check the car too, especially if something doesn’t seem right.
Conclusion
Whether you’re troubleshooting your own vehicle or inspecting a used car before buying, using an OBD2 scanner to look at DTCs, freeze frame data, readiness monitors, and code history can help you avoid hidden problems.
It’s better to know what’s really happening with your car than to face a big repair bill later on.
FAQs
How long should I drive after clearing codes before an inspection?
After clearing codes, you should drive your car for at least 50 to 100 miles under varied conditions (city and highway) to ensure that the readiness monitors reset and the car is ready for inspection.
Can a dead battery clear diagnostic codes?
Yes, a completely dead or disconnected battery can clear stored diagnostic codes, as it can reset the vehicle’s ECU. However, it will also reset all readiness monitors, which will need to be completed again.
Does clearing codes remove all historical data from the ECU?
Clearing codes will remove most active and pending DTCs, but some permanent codes or logs may remain in the ECU until the underlying issue is confirmed to be resolved by the car’s diagnostic system.
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