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When buying a used car, one of the main concerns is the vehicle's condition and whether it has hidden issues. It is important to note that the OBD2 (On-Board Diagnostics, Second Generation) system became the mandated standard for all light-duty vehicles sold in the United States starting in the 1996 model year.
OBD2 codes are crucial for identifying problems, but what happens if the seller clears those codes before selling?
Can you tell if someone has cleared an OBD2 code?
Signs That OBD2 Codes Were Recently Cleared
Clearing OBD2 codes before selling a car is a common tactic used by dishonest sellers.
By resetting the system, they hope to hide any problems that might show up during a diagnostic check.
Essentially, this gives the car a "clean bill of health" temporarily, making it appear problem-free. While it's difficult to prove with 100% certainty that someone has cleared OBD2 codes, there are a few signs to watch for.
While it's difficult to prove with 100% certainty that someone has cleared OBD2 codes, there are a few signs to watch for:
Check Engine Light (CEL) Behavior
If the Check Engine Light is off at the time of purchase, but turns on shortly after driving, it could be a sign that the codes were cleared.
Codes may have been cleared to hide issues that were not yet triggered but eventually will be.
Related Reading: Easy Ways to Check Engine Light Codes Without Car Scanners
Freeze Frame Data
Modern OBD2 scanners can pull up freeze-frame data.
This data is stored when an issue occurs and can show the exact conditions when a problem was detected. If no freeze-frame data is available or the data seems suspiciously recent, it could mean the codes were recently erased.
Emission Readiness Monitors
Vehicle computers perform a series of tests on critical systems, known as readiness monitors.
If the system passes, the monitor is set to "ready." When codes are cleared, the readiness monitors are reset to "not ready," which can be seen using a car scanner tool.
The system then starts the tests over, and if a problem persists, the same code will reappear.
If the seller cleared the codes before you bought the vehicle, the readiness monitors would likely show as "not ready," which could indicate a recent reset.
If you had a pre-purchase inspection done, the scan tool would likely show the monitors in this state, hinting that the codes were cleared.
Scanning History
Some advanced car diagnostic tools can show the history of codes, revealing whether any have been cleared. If the scanner shows recent resets, it’s a red flag that the seller might be hiding something.
Understanding Code Storage: Current, Pending, and Historic Faults
When a fault occurs, the vehicle's computer doesn't just generate a single code; it classifies and stores the information in specific ways. Understanding these categories helps a buyer discern the severity and history of an issue:
| Fault Code Type | Trigger Condition / Status | CEL Status | Storage Mechanism / Clearing Method |
|---|---|---|---|
| Pending Codes | Fault detected in a single drive cycle, but not yet confirmed as a permanent fault. | Not illuminated | Temporarily stored. May clear automatically if the fault does not recur in the next cycle. |
| Stored (Active) DTCs | Fault confirmed in two consecutive drive cycles. | Illuminated (Check Engine Light) | Long-term storage. Remains until manually cleared by a scanner, or after 40–50 consecutive fault-free drive cycles. |
| Historic Fault Codes | Fault was active but has been repaired, or no longer exists. | Not illuminated | Accessible only via OEM or brand-specific scanners. Stored as historical records in non-volatile memory; serves as a reference for in-depth diagnosis and helps buyers identify hidden issues. |
Code Retention Time:
Stored/Active DTCs are typically retained in the vehicle’s computer until they are manually cleared, or until the vehicle completes approximately 40-50 consecutive, successful fault-free driving cycles. Manual clearing is the fastest way to erase these fault codes and associated diagnostic data.
How to Detect Cleared OBD2 Codes Before Buying a Used Car
Use an OBD2 Scanner
One of the best ways to detect cleared codes is by using a bluetooth obd2 reader.
A diagnostic scan can reveal whether any codes are present and show whether the readiness monitors are complete.
Some scanners also provide historical data that could show if codes were cleared recently.
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Advanced Tools and Alternatives for Code Management
While using a standard OBD2 scanner is the primary method for detecting cleared codes, buyers and sellers should be aware of specialized tools and alternative methods:
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Brand-Specific Software (e.g., Forscan): Tools like the Forscan software are used by mechanics and advanced DIYers, particularly for Ford, Mazda, and Lincoln vehicles. This software, often paired with a compatible OBD adapter, can access deeper manufacturer-specific codes (including history codes) that generic scanners miss. Sellers using such software for deep resets must be aware that these specialized tools often leave traces that dealership-level equipment can detect (see FAQ 6).
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Clearing Codes Without a Dedicated Scanner: While not recommended as a long-term solution, some drivers attempt to clear basic codes by disconnecting the car battery for several minutes. This method forces a hard reset of the ECU, effectively clearing all codes and readiness monitors, exactly like using an inexpensive scanner. However, it also resets other system memories (radio presets, learned idle positions), which can be another subtle indicator of a recent, amateur reset attempt.
Check for Pending Codes
Even if the codes are cleared, there might be pending codes that haven't yet triggered the Check Engine Light.
These codes can often be found using an OBD2 scanner and may indicate underlying issues that haven't been fully resolved.
Inspect the Car's Maintenance History
Ask for the car’s maintenance records. A well-documented service history can provide clues about the vehicle’s condition and whether it has had recent issues. If the records are sparse or if the car was just serviced recently without any clear explanation, it could indicate a red flag.
Conclusion
When buying a used car, it’s essential to protect yourself by understanding the possibility of cleared OBD2 codes.
While it’s difficult to prove if the codes were cleared, by using a reliable car diagnostic scanner automotive, checking for pending codes, and paying attention to warning signs like the behavior of the Check Engine Light, you can avoid buying a car with hidden problems.
FAQs
Can an OBD2 scanner help detect cleared codes?
Yes, an OBD2 scanner can reveal whether codes have been cleared by showing the readiness monitors' status and historical data. Some advanced scanners may also show whether the system has recently been reset.
What should I do if I suspect the codes were cleared before selling?
If you suspect that the codes were cleared, request a pre-purchase inspection and ask the mechanic to use an OBD2 scanner. Look for signs like pending codes, "not ready" status on the readiness monitors, and any discrepancies in the vehicle’s maintenance history.
Can clearing the OBD2 codes hide serious problems with the car?
Yes, clearing the codes may temporarily mask issues such as engine problems, transmission failure, or emissions system malfunctions. A code reset doesn’t fix the underlying problem, and it will likely trigger again once the system detects the issue.
Can a dealership or professional mechanic detect if I recently cleared codes?
Yes, absolutely. While standard OBD2 scanners rely primarily on the Readiness Monitors showing "Not Ready" to indicate a recent reset, dealership and specialized independent mechanics use Original Equipment Manufacturer (OEM) diagnostic tools. These manufacturer-specific tools often have deeper access to the vehicle's onboard computer (ECU/ECM). They can sometimes access non-volatile memory or specialized fault counters that record specific system reset events and the number of times codes have been cleared, often with corresponding date and time stamps. If a seller is attempting to hide a persistent issue, a dealer-level scan is highly likely to reveal the recent clearing of codes.
What exactly happens in the car's computer when I erase OBD2 codes?
When you erase OBD2 codes, the vehicle's onboard computer (ECU/ECM) does three main things: 1) It clears the stored Diagnostic Trouble Codes (DTCs) and turns off the Check Engine Light. 2) It erases the Freeze Frame Data, which is the snapshot of engine conditions when the fault occurred. 3) It resets all Emission Readiness Monitors to a "Not Ready" state, forcing the car to re-run all diagnostic tests before it can verify that the issue is truly fixed.
As a seller, what is the best practice if my Check Engine Light (CEL) is on?
The best practice is full transparency. Selling a car with the CEL illuminated will significantly lower its value and erode buyer trust. A more ethical and effective approach is to have the issue diagnosed and fixed before listing the vehicle for sale. If you choose not to repair it, the most honest approach is to disclose the existing fault code and the known issue in the listing, adjusting the price accordingly. Attempting to clear the code without repairing the underlying problem will likely be detected by a smart buyer or mechanic, potentially voiding the sale.
