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What is diagnostic code 42 in a 1997 Jeep Grand Cherokee?

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Answered 2009-03-09 01:18:56

[The link bar feature is not available in this web] Ah, yes! The infamous TJ Check Engine Light; official designation, the Malfunction Indicator Lamp (MIL). If you have a TJ, then you have most likely had this stupid light come on one time or another. What does it mean?What do you do when it comes on? And, most importantly, how do you turn the blasted thing OFF ?!?!? There's been a lot of discussion on various newsgroups and e-mail lists about this "problem". Dirt Road Magazine has a page called "TJ Error Codes" at, which explains the procedure for determining the applicable "error code", and a table describing what each code means. For your convenience, I've included this same information below; but I've also included additional information on how the whole On-Board Diagnostics (OBD) system works as well. The Powertrain Control Module (PCM) (a.k.a., the computer or "black box") pictured to the left, is mounted to the firewall, under the hood, just above the battery. Its job is to monitor numerous different circuits in the fuel injection, ignition, emission, and engine systems. If the PCM senses a problem with a monitored circuit often enough to indicate an actual problem, it stores a Diagnostic Trouble Code (DTC)(a.k.a., an Error Code), in the PCM's memory. Diagnostic trouble codes that affect vehicle emissions will cause the Malfunction Indicator Lamp (MIL) (a.k.a., the Check Engine Light) to illuminate continuously while the engine is running. If the problem is repaired or ceases to exist, the PCM cancels the code after 40 warm-up cycles and the Malfunction Indicator (check engine) Lamp will go out. Note: Diagnostic Trouble Codes are the results of a system or circuit failure, but do not directly identify the failed component or components. A service technician can display a Diagnostic Trouble Code (DTC) in three different ways (you and I can use two of these methods, unless you have access to a "DRB Scan Tool"; I don't): # a two-digit number flashed on the Malfunction Indicator (check engine) Lamp

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