Too Lean code, misfire 1/3

Were those oem plugs?

 

"O2 sensor seems to be working fine (heated and bank 2)"

More clarity would be good there. There is only one bank, and it's bank 1. And there is only one upstream sensor which is in question here (b/c it's reporting the lean condition). It is a heated sensor - no separate heated sensor. Same sensor - with a heating element for when it's cold.

What - exactly - makes anyone say it "seems to be working fine"? Tech-y details are good. If that sensor is old, I'd just toss a new one in. Even if it's not the issue, those things don't last forever and do just decline over time - and are crucial for air/fuel mix and thus how the car runs and it's fuel economy. It's not a bad maintenance item.

Misfires can also give you a "fake" lean code - seems backwards like it should be rich. But O2 sensors only read the O2. If the mix doesn't blow up it's technically rich, but all the O2 sensor reads is the unconsumed oxygen.

 

Are there actually misfire codes, and of so which one/s? Just a P0300 (random / multiple)? Or specific cylinder/s? I'd also go ahead and clean the MAF. (It's quick & easy and can't hurt if you're careful not to touch the sensing wires with anything but MAF sensor cleaner). I'd also have a look at the readings out of the coolant temp sensor. Longshot, but it could be reading wrong, but not so out of spec that the computer would necessarily see it as an "error." I'm pretty sure the OEM thermostats are 185F. So you could compare coolant temp reading at cold start - should be approx ambient air temp, and then after it's fully warmed up it should be around 185ish - and, of course, the temp climb should be reasonably smooth and steady.

Before going too nuts, though, I'd throw in the O2 sensor and see what happens. I don't know the specific details for the Prius, but it's typical that the car starts in open loop, not using the O2 sensor info right away. Then once all warmed up, goes to closed loop using the info from the sensor for fuel mix. That's probably not too long though, just because they're heated O2 sensors so should be up to operating temp pretty quickly.

Throttle body cleaning is also never a bad idea...

 

The plug coils and then the injectors. I'm surprised your mechanic has not already done this. It's pretty basic for P0171, P03xx.

What do the fuel trims look like?

 

I'd swap the 1 & 3 coils w/ 2 & 4 and see what happens.

How hard is it to get to these injectors? I never looked. If it's easy and the misfires don't move w/ the coils then I'd follow with the injectors.

(Actually, if it was easy I'd already have said to check fuel pressure and compression. But there's no test port for the fuel system - dumb! And compression? IDK? Disable ignition & fuel and keep turning the key? I'm still just used to "normal" starting systems).

 

There's a feature built into the HV ECU to do exactly that. You activate a "compression test" in Techstream, and it keeps fuel and ignition disabled, and uses MG1 to spin the engine at a low RPM similar to a conventional car's starter cranking, so a conventional compression gauge gives you the right numbers.

If you don't have Techstream or that sounds like too much fuss, a leakdown tester allows you to get similarly informative results without cranking the engine (an air compressor is needed, though).

 

Excellent info on all counts (though I tried once and failed to get a Techstream version up and running. One of these days...)

As for the leakdown, the cylinder being checked needs to be at TDC on the compression stroke. Can this engine be turned by hand without causing any problems? I'd assume so, but I don't like to assume...

 

Sure can. In fact, it's small enough and low enough in friction that, with the plugs removed, you can turn it literally by hand.

I would always want to turn it in the normal direction (same as the forward direction for the wheels).

Double and triple check that the car is not in READY before grabbing it, of course.