It can be a relief to plug in your OBD2 scanner and see the words “No Codes Found” flash on the screen. You might think, “Great, no engine problems!” But sometimes, a “no codes” reading isn’t good news, especially when buying a used car. Here’s why a seemingly clean scan might actually be a red flag, and what to look out for.
One common trick some less-than-honest sellers use is to temporarily disconnect the car battery right before you come to inspect it. Why would they do this? Because disconnecting the battery wipes out the car’s computer memory, including recently stored diagnostic trouble codes (DTCs) and, crucially, the readiness monitors.
Readiness monitors are like system checks that your car runs to ensure all emission control systems are working correctly. These monitors need to complete their tests and set to a “ready” status before a car can pass an emissions test. This process takes time and requires specific driving conditions.
When the battery is disconnected, all these monitors are reset to a “not ready” state. If you immediately plug in a basic OBD2 scanner after the battery has been reconnected, it might report “No Codes Found” simply because the monitors haven’t had enough time to run and detect any problems. It’s not that there are no issues; it’s that the car hasn’t had a chance to diagnose them yet.
This is exactly what happened to one used car buyer. Using a simple, inexpensive OBD2 reader, they were told “0 Codes” on a used vehicle. However, after purchasing the car and using a more advanced scanner, they discovered the “codes cleared” time was very recent – suspiciously close to the time they had contacted the dealer. The advanced scanner revealed “not ready systems,” indicating a recent battery disconnection. The underlying issue, suspected to be a faulty catalytic converter, was masked by this tactic.
So, how can you avoid being fooled by this trick? Here are a few key indicators and tips:
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Check the Radio and Clock: If you get into a used car and the radio stations aren’t programmed, or the clock is obviously wrong (especially if it’s something generic like 12:00 or 12:01), this is a strong sign the battery might have been recently disconnected. Normal car usage keeps these settings intact even when the car is off for a while.
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Instant “No Codes” Reading: Be wary if your OBD2 scanner instantly displays “No Codes Found.” A legitimate scan usually takes a few moments to communicate with the car’s computer. An immediate “no codes” result, especially on a basic scanner, should raise suspicion.
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Invest in a Better Scanner: Consider using a more advanced OBD2 scanner that can show you the status of the readiness monitors and the time since the codes were last cleared. This information can reveal if the system has been recently reset.
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Test Drive and Rescan: If possible, take the car for a significant test drive, including highway and city driving, to allow the readiness monitors to run. Then, rescan the car. If codes appear after a proper drive cycle, it indicates the initial “no codes” reading was misleading.
Seeing “No Codes Found” on an OBD2 scanner isn’t always a guarantee of a problem-free car. Especially when buying used, be observant and look for clues that might indicate a recent battery disconnect. A little extra caution and the right tools can save you from buying a car with hidden issues and facing unexpected repair bills down the road.
