There’s a WHAT in the Prius?!?

by Ben N on September 21, 2018

The other day, our 2004 Prius quit working. When I opened up the battery pack to find the problem, I couldn’t believe what I found inside. It certainly wasn’t what I was expecting!

My wife had left the driveway and then pulled back in less than a minute later, traded cars, and left again. She said there was a big red light. I was very busy with my work right then, and we have a spare car, so I figured I’d get to it as soon as I could.

Today, I looked at the car. Sure enough – there was a giant red triangle with exclamation point. I keep a ScanGauge in the Prius. It’s a great tool for showing your instant fuel economy. It also lets you check error codes and even turn OFF the check engine light! (

I used the Scan Gauge to pull up the error code on the car. It was P3056, which indicates a Current Sensor Malfunction. Unfortunately, that current sensor is INSIDE the battery pack! Well, that meant getting out the socket set and going to work on removing the back seat, a bunch of trim, and the cover to get to the battery itself.

Once I got down to the pack, I pulled out the orange safety plug and set it to the side. Finally, I removed the sheet-metal cover that goes over the power contactors and houses the current sensor.

When I did, I saw a MOUSE NEST! MICE HAD GOTTEN IN THE BATTERY! That can’t be good! Nothing makes corrosion quite like mouse urine!

I started picking away at the nasty mouse nest when it got even worse. The mice had chewed away the wires! I could clearly see bare wires in the mass! I used my shop vac to get the rest of the nest out. At that point, it was easy to see how three wires were completely chewed away. And those were the exact three wires that went to the Current Sensor.

At least I clearly found the problem!

The other issue with the wiring is that the current sensor wires go to a single plug combined with two OTHER sets of wires. One bundle of wires goes into the upper part of the battery pack, but the last bundle of wires goes UNDER it! If I can find the right wire harness to order, I’ll still have to pull the battery pack to replace it!

I started looking through eBay and mail-order auto parts, trying to find the right name for these wires. After watching a YouTube video, I found out that the wires OTHER than those for the current sensor are for measuring battery pack temperature. I did locate a used wiring harness – at a cost of about $100. I placed the order, and it should be here some time next week.

In the mean time, we are down to just the one car. Which is fine, but next Saturday, we are planning a road-trip to a friend’s wedding in “Far, Far Away”!

Can I fix the Prius in time? Will we remember to bring the wedding gift? Tune in next week in this unfolding drama!

Also, did I mention I hate mice?

Until next time, stay charged up. Even if rodents chew your wires.


PS: It should go without saying, but please observe all safety around high-voltage systems. The DC voltage of this battery pack can kill. Only those with appropriate skills and training should be working on HV systems, and always using correct safety equipment.

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