Jump to content

Recommended Posts

I've been a Jeep enthusiast since 1997.  My first Jeep was a 1982 CJ8 Scrambler.  One YJ and 4 TJs, the current one being a 2004 Rubicon with 52,000 original miles and trail modified.   


I have been experiencing the famous P0301 trouble code very intermittently unless it was cold outside, then it was steady.  Rain or shine didn't seem to change anything.   I live in Minnesota where winters are cold.  Started with the obvious tune-up, copper plugs and the coil rail.  Battery connections, grounds, injector cleaning and replacement, throttle body cleaning. The works.   Could not isolate the misfire.  Started chasing wires.  No luck.   Many live data scanners,  no luck.  Compression tests and leak down well within spec.  Valves also seating and rotating well.  Became very frustrating.   Pulled the alternator, tested good.  The battery tested good ( the first time).  Wasn't sure what to do.


Finally, the other morning it was -17 degrees F.  The Jeep just didn't have the juice to start.  Thought that was strange.  Rain or shine it always started.  Jumped it and went to auto parts store.  Dead cell in battery.  Thinking back first good freeze, we experienced this and had the same test done.  I overlooked the issue and accepted its first passing test.   


No one on any of these forums makes mention of voltage or battery replacement or testing.    Well I replaced the battery.  Not a single misfire in a month.   I've wasted countless hours diagnosing this misfire, and it ended up being my battery.  Turns out they're pretty sensitive to low voltage. 


Thx for all the info on your site!

Link to post
Share on other sites
  • Administrators

Rubicon04...Welcome to the forums with your first topic post and valuable insight into the P0301 trouble code!  Thanks for sharing, others will benefit greatly...Your pristine TJ Wrangler Rubicon has so few miles that in my experience, I would be looking for something simpler than the laundry list of P0301 trouble code possibilities.  We're in an era where trouble codes are considered the last word, and frankly, many professionals waste time and money replacing parts associated with the P0301.  Since the battery is too basic to make the P0301 code list, troubleshooters wind up chasing the list of defects.  In this case, a basic item gets overlooked.  Here is a link to a very well done, comprehensive descriptive of the generic P0301 DTC: http://repairpal.com/OBD-II-Code-P0301.  Note that there is no mention of the battery condition in the list of possible defects.  Engineers assume that technicians will make sure the battery is in good condition before tackling a DTC problem.


The PCM on a Jeep vehicle needs 12.4V to function.  If the voltage drops below this point, there will be trouble—and trouble codes.  These codes may or may not be valid, some get triggered simply by the low voltage.  Let me clarify that if a battery cell is dead or dying, you may get a "normal" voltage reading on a static and unloaded battery.  The open cell will show up during cranking, where voltage drops substantially.  After cranking, the bad cell may stay "open" from the previous cranking load or open up intermittently under a variety of conditions. 


One such condition is the cold weather.  An approach that works wonders for me is the use of a Battery Tender or CTEK battery charger with a maintenance mode.  The absolute worst stress to a battery is when voltage drops for any reason (cold weather, the clock in the radio, setting for long periods of time, etc.) and you start the engine.  It may start readily at even 12.4V, but this is deadly low voltage for a battery. (Normal peak charged voltage for a healthy battery can reach 12.8V-12.9V.)  Upon startup, the alternator tries to compensate for this low voltage by shoving high amp current into the depleted battery.  This high voltage in a weak battery is the singularly worst source of damage possible.  By using a battery maintenance device, the battery has a full state of charge at cranking time, and the recovery requires substantially less amperage/voltage from the charge circuit.


Note: I use a Battery Tender (more recently a CTEK charging device, click here for my HD video details on the CTEK Charger) on each our vehicles: the '99 XJ Cherokee and the '05 Dodge Ram Cummins 3500 4x4.  The batteries in the Dodge Ram are the originals that came with the truck new in October 2004!  In the winter, I hook up the maintenance charger every time I park each of these vehicles.  The Dodge Ram batteries get maintenance charged year round, as this vehicle can set for three weeks at a time, winter means subfreezing weather—yet the engine cranks and starts perfectly while the batteries survive!  The Cherokee's single battery lasts a very long time as well, this is a daily driver ready to start in subfreezing weather that doubles as a moderate trail runner with a possible winch load.  The Cherokee does not require the maintenance charger in the summer or warmer months, the maintenance charger is connected every night of the winter! 


When having a battery tested, the optimal approach is to first remove the battery from the vehicle.  (This protects the PCM and other vital electronics.)   The battery should be "load tested" on a battery testing machine.  This will simulate a cranking load.  Another test, from my era of wrenching, is to read actual voltage per cell.  This helps pinpoint dead cells.  Think of a dead cell as an open in the battery's circuit.  It can take a simulated load to generate a dead cell symptom.


As Rubicon04 notes, it takes a cranking load or simply a deep freeze to create the load necessary for opening up a bad cell.  One of the most useful devices I've played with lately is the CTEK Battery Charger that has a variety of built in test features and even a "Recond" mode for de-sulfating a battery.  A dead cell is time to scrap (recycle) a battery; however, there is no cure for a dead cell.


When a vehicle has too few miles to start throwing catastrophic codes like a P0301, or worse yet the misfire code dances from one cylinder to another, it at least makes sense to follow Rubicon04's insight: remove and load test the battery(Load testing a defective battery can be very dangerous, as the gas emitted can be highly explosive hydrogen!)  A new battery may be the "fix" for a variety of engine codes and false alarms.  At least have the battery load tested before condemning all of the spendy P0301 components! 



Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Create New...