You've got an issue with both downstream sensors. When one shows up, these codes are almost universally exactly what they say they are. You need to replace the sensor. However, it's unlikely to have both spontaneously go bad, unless there was a specific cause. If you did something like drive through a field of brush or have mice making nests (look under the engine beauty cover and in the windshield cowl tray), the sensors are the issue.

Aside from that, the first thing to do is make sure the battery terminals are clean and tight. I'd probably swing by the parts store for a free battery and charging system test. After that, I'd throw a new downstream sensor (either one) on there and see if the code goes away. If it does, replace the other one. Just make sure to use a Denso sensor from a reputable source.

 

Thank you for the reply. The sensors pass the resistance test but the wire harness specifically between the EFI relay and the sensor plugs does not. The B+ line is apparently shorted to ground. Where I'm at not is trying to find out the physical routing of this wire so I can inspect the entire path.

 

I'm back at it and realized I had my leads in the wrong port in my multimeter so my readings were a bit off but it still appears there is a short somewhere.

I'm now getting 0.8 Ohm from EFI Relay pin 3 to Downstream O2 sensor Pin 2 (B+) which checks out good.

EFI Relay Pin 2 to body/battery ground is 138 ohm when both downstream O2 sensors are unplugged (should be less than 10k Ohm) per the manual. If I unplug all the ECU plugs I get 0 Ohms to ground. I don't know if this means the ECU is bad or if another component on the B+ line is shorting to something else. Any clue as to where all the connections to the B+ line are in the physical wiring harness so I can start undoing them one by one?

 

I took the intake manifold off and unplugged the fuel injectors, coil packs and every engine sensor I'm seeing in view and I've wiggled the wiring harness all over the place and the short to ground is steady at 138 ohms.

When I unplug the AC pressure sensor it jumps from 138 to 173 ohms.

I took the ECU cover off and there's no apparent problems.

Any ideas what to try next?

 

Did the ECU get wet due to a leaky sunroof, clogged cowl drains, or leaky windshield seal? I think IJ1 is the connector that plugs into the ECU.

 

I took the cover off the ECU out and the cover off. There are no signs of anything wrong or past exposure to water or elements.

What is the ground test you referenced? The pic you shared shows 3 ECU plugs but my manual has 4. I'd like to look up that section in the service manual to read more.

 

What is the ground test you referenced? The pic you shared shows 3 ECU plugs but my manual has 4. I'd like to look up that section in the service manual to read more.

01-37: INTRODUCTION - HOW TO TROUBLESHOOT ECU CONTROLLED
SYSTEMS

I think it's just a generic pic of ECU terminals, not specific to a Sienna. I found that page in my '05 FSM while doing a text search for the connector you asked about: IJ1. I found three instances, one of them related to the starter relay-to-ECU. But it looks like I was wrong. It looks like IJ1 is part of the front fuse/relay box. For example:

 

I paid for the online factory service manual and have been able to follow the B+ line from the relay pin #3. I narrowed down the short to ground to the J3/J4 connector. When its unplugged EFI relay pin 3 is 0 ohms to ground, when its plugged in today I'm reading 146 ohms to ground. Pin 26 on the male J3 plug when unplugged is 0 ohm to ground and the corresponding pin on the female socket is 146 ohms so next step is to follow the line to the ECU.

 

ECM connector E4 pins 1 and two of the plug when its unplugged is 0 ohm to ground. The corresponding ECM pins are 146 Ohm to ground. Appears my wiring is ok, but going by the factory service manual the conclusion of my test results point to a wiring harness. Does anyone have a clue what the resistance of these pins to ground should be?

 

Aren't E4-1 and E4-2 supposed to be B+? Why would they read 0Ω to ground? That doesn't seem right.

Edit: I'm probably talking out of my hat, but check this connector. The transmission wires go into E4 too. I'm wondering if it's corroded and is somehow grounding out adjacent pins.

https://www.siennachat.com/posts/478560/

 

Yes E4-1 and E4-2 are B+ so they should read 0Ω to ground. What is strange is if I unplug all the other ECM plugs the resistance between B+ and the ECM ground plug is much higher, I forget the number but it was multiple k ohm so technically it could be in spec per the service manual which is > 1k ohm (but the spec is testing from the harness with everything plugged in). Based on all this it would seem the ECM is enabling a ground path for B+. The question is if its a short in the ECM or if its another component wiring harness that is shorted that's connected through the ECM. I thought eliminated any possible ground short of the B+ line through what I did in posts #10 and #11.

I checked out the transmission plugs, I was hoping that would be it but they look clean and when I unplugged them I still get the same short to ground of B+.

I think I've unplugged all the sensors related to the engine except the stuff under the body behind the engine like the carbon catch can and fuel pump.

 

I hate to parts-shotgun but I'd be frustrated as hell if this were my van. I think I'd bite the bullet and order all four sensors for the P0038, P0058, and P0420 codes, install three of them, and see what happens. I'd probably even cheap out by buying all four 0₂ and A/F aftermarket versions for ~$50-$100 on Amazon just for troubleshooting purposes.