Usually, this is indication of an overly rich condition, usually caused by severe ignition faults or fuel injection that if left alone could damage an expensive catalytic converter. However, a FLASHING CEL indicates something severely wrong that could cause vehicle damage.
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These differ dramatically from the "solid on" CEL, in that if it comes on and stays on, it's an indication that something is wrong, and the driver should seek service at a convenient opportunity. The ECU/ECM constantly monitors the PID and emission conditions, and will eventually relent, given enough "clean" drive cycles.Īs an aside, there is a category of DTC that cause a FLASHING CEL. In most cases, however, a technician clears these codes after a valid repair to assure the customer that the repair is complete.
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The number of drive cycles required to clear an active CEL DTC depends on the fault and software implementation.
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If the condition that caused the fault is repaired, or simply no longer occurring (P0420 catalyst efficiency a classic example) the code will "clear itself" so to speak after a certain number of drive cycles without the fault reoccurring. This provides background to an astute technician, even when there is no current pending or logged DTC faults.ĭTC codes do NOT have to be cleared "manually". In addition, some Engine Control Units/Modules (ECU/ECM) have the capability of logging a few or dozens of "historical" fault codes, regardless if they have been repaired and/or cleared. These are true fault codes that have been promoted from "pending" status to a real-deal fault code, and they by OBD2 definition must set the CEL. The other category of DTC is "stored" or "logged". (This is accessed through OBD2 Mode 7.) The number of "drive cycles" required to promote a "pending" to a CEL depends on the fault, the implementation, and the vehicle. However, all vehicles have two categories of DTC: "Pending", which is a detected fault, but doesn't set the Check Engine Light (CEL, SES) until the condition is detected again a certain number of times. On my 1997 Subaru, data is limited - given that OBD2 wasn't a mandate until the 1996 model year. The history of such codes again varies with the implementation of OBD2, and quite likely how new the vehicle. On more sophisticated scan tools, Mode 2 "freeze frame" data can be displayed, which is invaluable data as it reveals the exact running condition at the very instant the DTC code was set. The simple "Pxxxx" DTC faults are accessed via OBD2 Mode 3, which is often the extent that simple consumer scan tools are able to display. These parameters cover everything from RPM, vehicle speed, O2 sensor data, mass airflow data, long and short term fuel trims, ignition advance, intake and coolant temperature, and perhaps dozens more. However, in most cases, a Diagnostic Trouble Code (DTC) is logged with a freeze frame, which is a complete store of all Parameter ID's (PIDS). What my 1997 Subaru logs (virtually nothing) compared to a 2015 Chevy Cruise are entirely different things. It really depends on the implementation of OBD2. I know you can clear codes using scanner tools, is this to suggest that faults would stay flagged on a system in perpetuity until you manually clear them? This would lead me to believe that the check engine light would remain illuminated even after repairing the root cause of a fault.
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Is that the full story with a fault, are there any other metadata surrounding a fault instance that can be looked up? How do fault histories work in vehicles? Are histories even recorded or are faults simply a binary thing, either on currently or off currently regardless of whether they were ever on at any point in the vehicle's life. One obvious aspect of the design is the use of unique codes specific to some particular problem. What exactly is the design around these fault codes as far as how they're recorded. I'm imagining some design similar to the Event Log used in computer operating systems but I may be totally mistaken on this one. I grew curious if these fault codes get logged somewhere with a date and time stamp of some kind at the time they were thrown. Just scanned my vehicle for the first time to check out the code for a Check Engine Light.