Stored Code

📖 YOUCANIC Automotive Glossary

History Code (Stored Code)

A history code, also called a stored code or past code, is a diagnostic trouble code that was previously active but the fault condition is no longer currently detected by the ECU. The code remains stored in the ECU’s memory as a record that the problem occurred, even though the system is currently passing its diagnostic monitor. History codes are extremely valuable diagnostic information because they reveal intermittent problems — faults that come and go depending on driving conditions, temperature, vibration, or electrical load. For example, a crankshaft position sensor with a failing internal connection might only lose signal when the engine reaches a certain temperature, triggering a code that later becomes a history code when the engine cools down and the sensor works again.

Many DIYers make the mistake of ignoring history codes because the check engine light may have turned off on its own, but these codes point directly to developing problems that will likely become permanent failures. Most ECUs will automatically turn off the check engine light after three consecutive drive cycles where the monitor runs and passes without detecting the fault, but the history code remains stored for a manufacturer-defined number of drive cycles (typically 40-80 warm-up cycles). Your OBD2 scanner should display these separately from active codes — look for labels like “History,” “Stored,” “Past,” or “Matured” depending on your scanner’s terminology. When diagnosing intermittent issues, history codes combined with freeze frame data tell you exactly what was happening when the fault last occurred. Always document history codes before clearing them, because once cleared, you lose the information about what intermittent problems the vehicle has been experiencing. If you see a history code for a component that you know sometimes acts up, that code is confirming your suspicion and telling you which component to replace before it fails completely.