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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.
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.
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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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.
