2010 Prius won’t start. UO100 code

Checking for a short circuit in a wire is performed with consumers disconnected.
You use a multimeter in the current measurement mode, but it is necessary to use it in the resistance measurement mode. Measurement between the wire and the car body. Low resistance measured as zero ohms or 10 ohms is bad, high resistance of several megaohms is excellent.

 

If there is a short to ground it will be intermittent so in order to find it with continuity testing on a meter you would have to keep an eye on it while wiggling stuff around. I think a test light from battery + to the component side of the fuse (with the fuse out) might be an easier way to run the short to ground wiggle test. I don't think you have to unplug anything.

The current reading shown is useful and tells me that nothing appears to be drawing excessive current in that condition. So I would be leaning towards a intermittent short of some. I would first want to do a visual check of anything associated with that B+ wire. Any signs of rodents? Does it feel like the fuse is more likely to blow over bumps/turns or does it ever happen while cruising? Then probably move to the wiggle test.

 

The fuse burns out, this indicates a short circuit. Either the wire shorts to the case (negative terminal 12V), or one of the consumers has internal damage and it causes a short circuit. When checking the resistance between the wire and the car body, high resistance means that the insulation is good, low resistance means that the insulation is bad and there is a short circuit to the case.

 

You need to disconnect all consumers of this fuse, not just one connector. Look at the diagram.
If you disconnect only one connector, you will not understand why voltage appears on this wire.

Positive 12V is supplied to one fuse contact, and a branching line goes from the second fuse contact to six consumers.
Until you disconnect all six consumers, checking the wiring harness for a short circuit will not give the desired result.

 

Watch this video.

imagine that the fuse is a switch for a multi-lamp chandelier.
Until you unscrew all the bulbs, the light will be on.
Therefore, you need to disconnect all consumers from the fuse power supply. This can be done by disconnecting the connectors of the units and sensors that I indicated above in the message #19.

 

Remove fuse. Use test light from battery NEG (with car on) to figure out which side of the now empty fuse socket is the voltage source (light will glow) and which side is the component side (light won't glow). Now hook up the test light to battery POS and touch the test light to the component side. It shouldn't be glowing because it's a B+ line and doesn't have a connection to ground. Now you can wiggle stuff around and mess with stuff trying to get the light to glow which will tell you that you have a short to ground.

Am I an expert? No. Have I tried this personally on a Prius? No.

I don't quite get MAX's logic here (that video is for a parasitic draw, which is different). I don't think resistance checks are typically the go-to method to find an intermittent short to ground. If the short was easily identifiable by low resistance to ground then the fuse would be blowing right away each time. Since it's not, you'd still have to wiggle stuff around and watch the meter at the same time.

I could be totally wrong, so you'll have to weight with your own knowledge about which technique to choose.

 

If you think it's happening as a result of trying to run an EVAP certification then I would certainly be considering the canister pump module or purge valve as suspects based on my wiring diagram.

 

If the wiring has a short to ground and the test light is hooked to battery POS then yes.

If it's really blowing consistently when attempting to run an EVAP check then that's the lead you should follow. If I had a donor car I would swap the Charcoal Cannister, which I'm assuming has the 'Canister Pump Module' (as per the wiring diagram), but I have not confirmed this. Purge Valve is also on that circuit but seems less likely to fail in that manner.

 

Also keep in mind that the probable cause may not even be the unit itself, but rather oxides in the unit connection connector, which create a parasitic chain in the overall consumption of this line.

 

Did it pass the EVAP check?

 

The fuse doesn't blow, that's good.
Are there any other problems?