Check Engine Light — Facts

May 8th, 2017 by

Perhaps the most common across-the-board issue with modern Porsches is the “check engine” light (CEL). The CEL is the indicator light for the on-board diagnostic (OBD) system of the engine control unit (or DME for “digital motor electronics” in Porsche-speak). Early, OBD-I systems (late 1980s-1995) were only capable of monitoring basic DME and engine sensor functions, and would only store fault codes and illuminate the CEL in the case of a complete sensor failure or an open or short circuit in the DME or a sensor wiring circuit.

For the 1996 model year, the United States Environmental Protection Agency (EPA) mandated the implementation of OBD-II protocols for all light vehicle engine management systems. OBD-II standards stipulated that countermeasures be taken to protect the catalytic converter(s), including monitoring of catalytic converter function itself (via additional oxygen sensors) and on-board misfire detection (by monitoring crankshaft speed and crankshaft to camshaft correlation to determine which cylinder is misfiring). Additional emission control systems such as the fuel vapor recovery/evaporative emission control systems and secondary air injection system function were also required to be monitored.

Therefore, a CEL on an OBD-II-equipped car is a much more common occurrence, as anything that could potentially affect exhaust emissions will cause fault codes—this could be due to a slight vacuum leak in the intake manifold to degradation of an engine sensor to a loose gas cap! In most cases, a steady CEL without any noted change in engine running is not an emergency, but a warning to take the car to a Porsche specialist as soon as possible. A flashing CEL indicates an active cylinder misfire that could possibly cause catalytic converter damage; if this is consistent, a tow to the shop might be prudent.

Generic OBD-II fault code readers are readily available these days, but they only fully display general OBD-II fault codes—any Porsche-specific fault codes will come up as “unknown.” A Porsche-specific scan tool will provide the proper fault code description and freeze-frame data, along with live engine running data to assist the technician in diagnosing the problem. It must be said that a stored fault code does not usually point directly to a component at fault; it merely provides the technician with a clue to the general problem area. Porsche publishes troubleshooting charts for most fault codes, but many experienced technicians develop their own shortcuts to help zero in on the problem.

Once the problem is determined and the repair is made, the fault code is cleared, and a series of “short tests” can be run using the scan tool to force the OBD-II readiness monitors to run to verify the repair (these on-board monitors normally take at least two complete driving cycles under specific driving conditions). In the case of a 2010 or newer Porsche, the fault codes are “permanent” and cannot be manually cleared with the scan tool (this was mandated by the U.S. EPA in an attempt to thwart DIYers from simply clearing fault codes with generic scan tools in an attempt to bypass emissions tests). Permanent fault codes will not clear until the relevant OBD-II monitor is run, or via special procedure with a factory-level PIWIS-II (Porsche Integrated Workshop Information System II) or PIWIS-III (aka PTG III) scan tool.

Porsche Redwood City has the diagnostic equipment and factory trained technicians to properly diagnose your check engine light or any other warning light. Call for a service appointment today.

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