Russ' Do It Yourself Home Workshop

Finding Fixes to Just About Anything and Everything

Replacing the Inverter Assembly on a 2002 1st Gen Prius

Posted by Russell Wright on September 18, 2011

As my wife and I were driving to Lowe’s the other day in my 2002 Prius, it suddenly started to shudder and run very rough.  We weren’t but about a mile or two from home, so we immediately turned around and limped home.  After we made it home, the car wouldn’t even start…it would simply shudder when starting was attempted. 

I went to the local Auto Zone and “borrowed” one of their code readers and found P3125 – Bad Inverter.  A quick search on the internet and I found the price for a dealer replacement was about $4500.  Ouch!

After further reading I also found that it may not be as simple as replacing the inverter, as P3125 can also mean other things.  So I decided I’d take it up to the Toyota dealer and let them diagnose the problem to see if our diagnoses agreed.  I got my brother-in-law to hook me up behind his Toyota Tundra with my tow strap and haul me the 8 miles to the dealership in Richardson.  $54 later, the dealership confirmed my diagnoses and quoted me about $4400 to replace it.  I asked if they would consider using my salvage part I purchased for $350 and they said, “No, too much liability.”  OK, I thought, I guess I’m on my own…

So we hauled it back and I finally got a round-tuit yesterday.  Since I couldn’t find a good procedure that anyone had documented online, I decided I’d document it and take a few pics along the way.  Here’s my result.

Disclaimer:  I R an electrical engineer, so I am comfortable working around electricity.  If you aren’t, perhaps you should befriend someone who is.  Engineers need friends, too!  Also, anywhere there were big orange HV cables I always checked the voltage between all connections with my meter before touching anything with a wrench.

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Before you start working on this car where you’ll be dealing with the high voltage (HV), you need to disable all the power and let it sit a few minutes to insure all the HV capacitors have had time to bleed down.  I’ve heard many time tables on this, such as “let it sit an hour,” but the Toyota dismantling manual states five minutes, so I figured by the time I got to working on that part plenty of time would have passed. 

First, disconnect the 12v “auxiliary” battery (located in the trunk) and pull the safety plug on the high voltage.  I disconnected the ground (-) battery cable.  Here are a few pics in the trunk showing the location of each.

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Pull the lever down to unlock the safety plug.

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Me pulling out the HV safety plug.  Hmmm, perhaps I should’ve worn insulating gloves (and a bunny suit).

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Now drain the inverter coolant.  It’s kinda’ scary because you might think you are going to drain the transmission fluid.  Oh, wait!  You will if you remove the wrong plug!  Don’t do that!  There are others that have much better photos of this.  Check out YouTube.

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Now you can remove the hoses to the inverter cooling system.

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As I started this process, I did things in the wrong order (of course) and found there were a couple of bolts I couldn’t get to.  I sent a text to the nice folks at Luscious Garage asking how I get to the bolts and they texted back (amazingly!) to remove the windshield wiper cowl.  Duh!  Should’ve done that at the very beginning.  My loss is your gain!

Remove the seal.

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The windshield wipers are held on with nuts that should be pretty easy to remove.  After the nuts are off, remove tension on the windshield wipers by pulling them away from the glass and see if you can’t easily remove them from the shafts. 

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The cover is essentially held on with two phillips screws (one on either side) and the fasteners that snap in the rubber seal.

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You do have to remove both sides of the cowl cover. 

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Once you get all this windshield wiper stuff out of the way, you can remove the cowl pan it sits in.  I didn’t get a picture of this for some unknown reason, but it’s pretty easy.  It just takes 5 or 6 bolts and you lift it out.  Be careful not to cut yourself (I did) on the sharp edges.

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Once you get the connectors out of the way you can get to the two gray connectors on the back and have easier access to the HV cable connectors (which can be somewhat difficult to wiggle loose, but be patient).

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22 Responses to “Replacing the Inverter Assembly on a 2002 1st Gen Prius”

  1. [...] [...]

  2. Randy Wright said

    Good job, Russell. I’m proud of you. Daddy

  3. [...] [...]

  4. [...] [...]

  5. Tim said

    can the old inverter be recycled?

    • Russell Wright said

      Not sure. I was wanting to find a place that could test it and possibly repair it…reasonably, of course!

  6. Abbey said

    we followed your instructions to the letter and then did it backward when we put the new one in and now the car won’t even turn over. Electrics in the car work but nothing happens under the hood. any ideas?

    • Russell Wright said

      Mine turned out to need a new traction battery.

      • Art Perini said

        I have the P3125 code on my 2001 and looking to replace the inverter myself. Already work with high voltage, have repaired wire harness problems in the traction battery pack, big hand problem with accelerator, and others.
        1) Looks like you replaced both the traction battery and inverter. Were they both bad at the same time? Or was only the battery the problem in the first place?
        2) Is there a good source of reliable inverters you know of?
        3) Toyota and non-Toyo mechanics keep saying don’t do it yourself because you have to re-program the system after the exchange. Was this true for you?

      • Russell Wright said

        I believe I replaced the inverter (purchased it off eBay from a salvage yard) and really didn’t need to. However, I don’t have a way to test the inverter I removed. I think my problem was simply the battery. You don’t have to reprogram anything. It figures everthing out in a few hours of operation and all has been well for the last 6K miles.

        If anyone knows of a place that will test an inverter, let me know!

  7. Octavia Lappia said

    Hello Russell,

    I have a 2002 prius with a code of P3125 and also looking to replace. I will like to know after you’ve replace the donor part, did your prius start few hours later without having to do anything extra to it? or

    • Russell Wright said

      It started fine after replacing the inverter. However, I think my probably was really batteries because in a couple of days I got another code which indicated batteries. I ordered a rebuilt from Involt, replaced it, and it’s been working fine for over 6K miles.

  8. kim said

    I had to have my inverter replaced in my 2010 prius… it was under warranty, luckily…. but… I have noticed that my gas mileage has been affected greatly… use to get about 48-50 mpg and now am getting about 43-44… anyone else experience this problem? Thanks!

  9. Josh said

    Do the part #s vary among the different model years in each generation or just among the generations?

    • Russell Wright said

      I think there are a couple of part numbers in the first generation Prii…you need to be careful, as I don’t know if they are interchangeable.

      1st gen definitely different than 2nd gen.

      Russ

  10. Wayne Hyde said

    Thanks for doing this and posting the procedure. Really helps-I’m sitll diagnosing my P3125 and P3130, but it my be a bad inverter, or it just got too hot today,…

  11. I have a 2001 that got a P3125 on a hot day and now does not even attempt to turn over the engine, and I just got a an entire donor vehicle that’s been rolled with a good battery and inverter.

    If any of you that posted previously still have inverters, I would like to attempt to devise some sort of inverter test process, or at least have some to take apart and reverse engineer them, so please let me know if you have one that’s failed.

    • Russell Wright said

      I would like to know if there is a place that can test/repair the inverter. It seems like a much better solution.

  12. LARRY OISTAD said

    iS IT A GOOD IDEA TO REPLACE THE ELECTRIC PUMP FOR THE INVERTER WHILE THE UNIT IS OUT?

    • Russell Wright said

      I have mixed emotions about replacing items that are working. Replacing the pump is not horrible, but it does take some work to bleed it. On the flip side, many would argue that you don’t want an failed pump to allow your inverter to overheat so, if it’s getting old, it might be the right thing to do.

  13. mike said

    Russell,,

    I if the Dealer told you It was the Inverter how come they did not say anything about the Battery, My friend has his up at the dealer and yes $4500.00,,, we want to try to do it and I know I can, But this makes me nervous if they told you the inverter and it was the battery

    • Russell Wright said

      I’m convinced there are not many dealers who know how to work on these. They are old and not enough of them for the dealers to worry about.

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