Super Weird 02/Emissions issue? NEED HELP!

Okay guys I recently picked up a 1999 4Runner (Cali emissions - at least it says so under the hood) with 150k miles. I got a good deal on it because it needed some basic work, but was also throwing a P0420. I figured at this mileage, even if I needed to replace all 02 sensors AND both catalytic converters, I'd be OK.

So I started with a tune-up. Plugs, wires, PCV valve, fuel filter, etc. All that really seemed to wake the 4Runner up, but it was still throwing that code! I then replaced the rear downstream sensor, then the A/F upstream sensor. All original Denso sensors. I'm watching both S1 and S2 voltage outputs and am noticing some very strange things going on.

When warmed up, S1 (upstream) will hover around .655v, and will stay there at idle under driven where it'll fluctuate between .63-.67 when driven normally. If at WOT, I can get it to swing up to .7 then back down to .3, then it'll settle around again at .655v or so.

S2 is all over the place, always. It'll bounce between .2-.8 and every range in between, whether WOT or idling.

There is a very real possibility that both cats need to be replaced, and I'm OK with that - but I figured I'd start with the sensors and see if changing them helped out at all. Based on the research I've done, S1 needs to be around 3.3V, and S2 can bounce around between 0v-1v, but not as much as it currently is.

Any ideas? I'm pretty handy and know my way around these trucks pretty well. Never had an 02/emissions issue like this before, let alone on such "low" mileage. Thank you all!

 

Have you unplugged S2 and inspected the connector? Clean with a solvent for electrical connectors, and plug back in and see how it does.

If that doesn't help, inspect the wire harness from the sensor as far as you can forward to make sure there is not a problem with the insulation on the wire.

 

What do your thermal measurements for the cats say? They should be cooler upstream than downstream.

Did you contact the dealer with your VIN to get the correct sensors\part numbers? Do NOT rely on the interwebs or big box stores to know what you need.

You sure the hood is original to the car? I've seen that story also play out.

 

Yup, visual inspection checked out. Receiving connector was clean with no bent pins/anything like that.

I’ll check wiring harness further when I can - are the voltage readings odd enough you’re concerned about a potential wiring issue?

Yup, I’ve checked cat temps before and after each one. Temps drop 50-100 degrees after each cat, so I know they’re somewhat clogged and could stand to be replaced.

To be honest about the hood, I’m not sure. It’s definitely the same paint color. I do believe it’s the original hood though.

The original AF sensor I removed physically matched the one I replaced, so I do believe I have a cali emissions 4Runner. Front was an AF sensor, rear was an 02 sensor.

Any ideas on voltage? .655v is definitely low for an AF sensor of this vintage right?

 

Bump - any ideas on why my upstream A/F voltage reading hovers around .655v or so??

 

If temperature drops after each cat .. the cat is bad.

They are supposed to raise the temperature due to catalytic reactions.

 

Yup. This is on my to-do list.

About my A/F voltage - any ideas??

 

https://www.tirereview.com/troubles...xt=Watch the voltage reading for,/F sensor(s).

Maybe this?

Another point that can trip up an unwary technician is that the A/F value displayed on a scan tool can be misleading. Many scan tools with “generic” OBD II software automatically convert the PCM’s A/F sensor voltage output to a more familiar 0 to 1 volt scale like that of a conventional O2 sensor. If you are not aware of this fact, and wonder why the voltage reading for the A/F sensor PID seems unresponsive, or does not change as much as you would expect when you create a lean or rich fuel condition, you may wrongly conclude that the A/F sensor is bad.

 

That’s a great idea - I have another scan tool I can use to read voltages. I’ll check back in later today!

 

Welp, interesting news.

If I unplug S2 (downstream 02 sensor), voltage drops to zero when running - duh.

If I unplug S1 (upstream A/F sensor), voltage STAYS THE SAME at .660v

Any ideas? Bad ECM/PCM?

 

Any thoughts on my last post?