P0300 P0480 Fans Run Until Battery Dies Yukon XL/Suburban/Silverado/Sierra Chevrolet/GMC

FREE DIY temporary fix, Expensive 100% permanent fix.

 This is what worked on our 2005 Chevrolet Suburban.  Your truck may be similar.

In your engine bay, just inboard of the battery (toward the engine), there is a curvy plastic box with lots of big cables going to it.  That is your engine computer (PCM).  Pull off the plastic cover on the front, by the radiator (it only snaps on).  It takes one small socket wrench (9/32″ I think) to unscrew the bolt holding the connectors to the computer.  Pull the connector nearest the radiator.  You will see blue plastic pieces on the connector.  This is the blue connector.  These blue pieces are clear so you can see the pins inside the connector.   Look at the lower end of the connector.  It will be full of corrosion.  Mine looked like there was mud inside.  Look in the mating connector on the Powertrain Control Module and see that it is also corroded inside.  That is your problem.

 photo enginebay1_zps119af7bc.jpg

 photo pcm_zps633d6696.jpg

The BEST fix is to replace the computer and wiring harness, which will set you back closer to $2000 than $0.  The fix to get your truck running involves disassembling the connector and cleaning out the corrosion one pin at a time, as well as cleaning the connector on the computer.  Open the computer itself and make sure it does not have water inside.  If you don’t know how to deal with static-sensitive electronics, leave the opening of the computer to someone who does, or you risk an expensive computer-killing static shock!

 The sequence of events:

First there was a check engine light, code P0332 (rear knock sensor).   Then there was running like [deleted] and barely able to drive down the road, with a flashing check engine light.  The transition took about 20 seconds from runs-great to runs-barely-and-smells-like-gasoline.  Sometimes on a wet/rainy day when the truck seemed to be running well, the truck engine would die like you had turned off the ignition by removing the key (while driving down the road).  This was unsatisfactory but it would always start right back up again.  This was likely from shorting out the crank sensor.

Oh, and P.S. the radiator fans worked normally, except that after the truck was off.  Anywhere from 11 seconds to maybe a couple of hours after the engine was stopped, the engine cooling fans would start turning at low speed until the battery was too depleted to turn the fans any longer.  When the truck was put on another car’s jump-start cables the fans started turning within a minute.  To prevent draining the battery totally, the free short-term fix was to pull the fans’ 40 amp fuse.  This did nothing for the running-like-poo symptom, however.

The engine computer was setting codes P0300 (random misfire) and P0480 (fan control circuit).  The bottom inch or two of the blue connector has wires for half the fuel injectors as well as the Crankshaft Position Sensor and the radiator fans.  The conductive slurry of corrosion in the blue connector was cross-shorting out the whole mess.

Disassembling the blue connector was quite a job, and I literally went to a school to treat corrosion in electronics – do NOT be afraid to ask for help on this job from a good local electronics tech!

The P0332 was legit, and if you do a bit of web searching, you will find that GM issued a TSB for this problem.  My rear knock sensor was stereotypically rusted and not-working, plus the knock sensor harness looked janky so I replaced that while I was in there, along with the other sensor.  This requires removal of the intake manifold, so it is either an $800+ job or $40×2 for sensors, $50 for the harness, and if the intake manifold gasket is in good condition and you have the right chemicals and tools (including torque wrenches!) that’s all it costs to DIY.

 The moral of this story: NEVER power-wash your engine bay.  Ever.  Power-washing puts water on top of the engine and rusts out the knock sensors, and power washing puts water in the PCM connectors.  Wipe it off by hand, or leave it.  It’s a truck, not a show car (unless it’s a show car, in which case trailer it!)

13 thoughts on “P0300 P0480 Fans Run Until Battery Dies Yukon XL/Suburban/Silverado/Sierra Chevrolet/GMC

  1. Hello:
    You have the knowledge and experience to tell people who already have years of knowledge in the repair industry. Your information has been a great help to us on my personal Suburban.
    We removed the PCM and found a lot of corrosion on the blue connector and computer terminals. We meticulously cleaned each pin and connector as well as taking the computer apart but found no water inside. This fixed the problem for several weeks but yesterday while sitting in the parking lot at Walmart with the engine running and the A/C on, the engine shuttered and died. I restarted the engine but it died again. We finally had to turn off the A/C in order to keep the engine from shutting down. What are your thoughts on this problem? and by the way we have a knock sensor code with check engine light on.

    James Tyson Sr.

    • If your connector has corroded again, then you didn’t correctly repair the corrosion. That’s easy enough to check. The knock sensors quite likely are bad if they haven’t been changed. Other than saying it sounds A/C-related, I don’t know what to tell you for your problem.. I recommend starting a thread on a suburban specific internet forum or maybe posting a thread on https://www.reddit.com/r/MechanicAdvice/.

  2. There was no A/C problem and the problem is fixed. People need to be advised to buy a spray can of CRC- QD electronic connector clearer for this project. Most parts stores carry it and even Walmart has it. Some cleaners seem to draw moisture when drying and contaminate the connecters all over again.
    I hope this helps,
    James Tyson Sr.

  3. David,

    Thank you for this post. Do you have a tutorial or pictures of the blue connector while cleaning? Did you do the cleaning while it was still in the engine bay or can it be removed?

    Your post described exactly what I have going on with the exact same vehicle.

    I don’t have mud in my blue connector but there is corrosion on the bottom few wires.

    Thanks for your post!

    • No pictures, sorry. I wasn’t even in a mood to be doing the work in the middle of the night but we needed the truck to be up and running! I hate to sound like some kind of a snob, but I’ve been doing electronics for a while, including specific training for dealing with corrosion, and know how to *not* completely mess this up. If you think this is something you need pictures for, either you need a lot more than pictures, or you need someone else to do the work.

  4. Thank you very much! This is the most detailed and informative answer/response that I have found. My 2006 Chevrolet Silverado 1500 4.8Ltr V-8, with 203,000+ miles on it (my first brand new vehicle) has just recently (yesterday – 11-10-2017) started with the electric fans coming on and staying on, after it has been turned off, doors locked, alarm set, and the keys with me (over 600 miles away)… I’ve been reading all morning trying to find trouble shooting ideas before I start taking anything apart or replacing relays or anything else. I’m no mechanic, by any stretch of the meaning. Just a poor man trying to survive and take care of a family in this mean and crazy world…

    Everything you’ve described and explained has been almost EXACTLY what has been going on with my truck.

    Last week it also gave a check engine code for the knock sensor/sensors error. I haven’t replaced these or the harness yet, but that’s next to do because of age and mileage I’m sure it needs it. This code showed up one day after my water pump went out (truck did not overheat, to my knowledge). I replaced it along with the thermostat, radiator hoses, and I even had both serpentine and A/C belts replaced at the same time. I paid my daughter’s fiance, a young 24 year old mechanic, that’s worked at the local Toyota dealership for a few years now, do all this work. He did good work with no issues since. What he also did, when he finished the job, was take it to the dealership and “clean” the motor… I can only assume that he power washed it. Which I’ve never done, and NOW know, to NEVER, EVER, power wash your engine…

    So, after reading your post, without looking or taking anything apart, yet, I can pretty much guaranty that corrosion/moisture in that blue box is going to be my problem… My $2,000+ problem… And of course, money is an issue right now. Hence the knock sensors not being replaced yet – after the almost $500 total water pump job… “Domino Effect” has been activated!

    Thanks again for your post and help. I’ll try to remember to follow up and update this after I repair/replace EVERYTHING… I love my truck and will never get rid of it!

    Troy – 44 year old bad timing magnet…

  5. 2005 tahoe. fantastic description and matched my symptoms and blue connector condition. One difference- found #7 fuel injector was stuck open, causing engine to run like shite. heavy gas smell and gas running down back of engine… disconnected fuel injector at work parking lot until i could get at connector. bolt head- 7mm socket fit perfectly. disconnected and water came out (thimble amount) and lower 10 connectors were corroded. cleaned and all symptoms went away. saved me $$$$$$$$$$$$$$- thanks

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