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JK Check Engine Light - P0300

One morning I slid behind the wheel of my Jeep, slid the key into the ignition, and tried to start the engine.  I was met with the ominous clicking indicating a low battery.  And, the dash was ablaze with a series of warning lights.

First step, get the engine started.  I carry an emergency battery pack that provides the necessary power to start a vehicle engine.  Within minutes, the engine was started and running very rough.  The dash warning lights were reduced to two, the Electronic Throttle Control warning and the Check Engine Light.

The ETC warning was a simple fix with a few key clicks and timed pressing the throttle pedal.  While that warning light was quickly cleared, the Check Engine Light remained on and idle remained very rough.


My INNOVA 5310 code reader revealed a stored trouble code - P-0300.  By definition, that code indicates multiple random mis-fires. The list of possible causes was long, some easy and some not so easy.  A series of repairs were in order, starting with a new battery and tuneup.

A new battery (Optima Yellow-Top) was installed.  Great.  No problem with engine starting now!  But, the P-0300 CEL was still present along with the rough idle.

The 2010 Jeep JK has the 3.8 liter V-6 engine.  At 92,000 plus miles, the mileage was approaching the 100,000 mile recommended tune-up point.  I prefer to stick with OEM recommended replacement parts, in this instance, Champion Double Platinum Spark plugs, Chrysler plug wires, and Chrysler Ignition Coil.

The 3.8l engine is compact with limited access space for working with three plugs per side.  The ignition coil is located on the drivers side. The OEM plug wires are identified with a number corresponding to the associated plug and its connection to the ignition coil.  The three wires for the passenger side feed from the coil behind the engine block.  The wires are supported with a plastic loom cradle that becomes brittle with age and heat.  During the tuneup, this plastic loom was replaced.

The driver side spark plugs were relatively easy to remove.  It does help to have socket extensions (long) that have a “wobble” or a u-joint adapter to allow for angle movement around that many obstacles and odd angles.  Leverage extension may be necessary to initially loosen the plugs.  When inserting new plugs, make sure plugs and thread by hand to ensure they do not cross-thread in the block sockets.  Plugs do need to be snug, not multiple foot-pounds tight.

The passenger side was challenging to access the plugs.  Some DIY instructions called for removing the inner fender lining and others resorted to removing battery and fuse box.  I was able to access the plus without those extreme measures.  Again, long socket extensions with “wobble” joints and some type of leverage.

My toolbox contains a very handy Tite-Reach Universal Extension/Extender for hard-to-reach areas and odd angles.  That tool proved to be just the socket adapter to provide necessary leverage with odd angles of access to remove the spark plugs.

With all plugs, wires, and coil replaced, it was time to start the engine.  Much to my dismay, the rough idle and P-0300 code returned.  As noted, the list of possible corrections is long with an increasing level of complexity, most dealing with fuel delivery issues.  Pressed for time, I opted to schedule shop time for dealer repair.

Dealer repair diagnosis confirmed the P-0300 trouble code along with three outstanding recalls.  One recall, Powertrain Control Module (PCM), was inadvertently programmed with incorrect software.

The PCM is the computer that controls most of the Jeep’s essential systems. The PCM controls the engine timing, the fuel to air ratio, the transmission, and the anti-lock brakes, along with dozens of other smaller functions.

Once the PCM was re-flashed with updated software, the P-0300 was cleared and engine idle was smooth. As the fault code was cleared by applying the factory recall, shop diagnosis costs were waived. 

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