hopk1719

I know this code has been talked about on the forum a little bit. This code was thrown on my 04 gx470 (112,000). I reset the check engine light buy unplugging battery. It pretty much came right back on. After trying to reset a few time, I replaced 02 sensor downstream driver side. Light went right off on start up. No check engine for about 1,000 miles. Now check engine back on with same code? Is this just a coincidence with O2 sensor keeping light off for a month, and my Cat is bad as well? Any insight would be appreciated.

chiph9

A downstream sensor code is telling you there's a problem somewhere upstream. Check for other codes, check that the catalyst is still good (not broken into chunks), check for a cylinder that is running rich (bad spark plug, bad ignitor, etc).

Chip H.

hopk1719

Thanks Chip. I just bought OBD reader to look at it a little more. Both upstream A/F Sensors had been replaced by Lexus before i bought it. I know if I take to dealer they will tell me to replace Cat. Everything seems to be running absolutely fine.

sheikhmax

Factory Manual for Troubleshooting.

Check the Monitor Description for DTC.

BlackCat81

It needs a cat.

hopk1719

You sure it is the cat? Should I have replaced both downstream o2 sensors at the same time? I only did bank 1, or does it not matter?

BlackCat81

It doesn't matter only doing one O2 sensor. You can try installing a spacer on the rear O2, it should help slow its response and trick it to thinking the cat is doing a better job. It's a bandaid, but it could work for you.

chiph9

If you don't have a scope to look at the waveforms per the diagnosis PDF that sheikhmax attached, check the sparkplugs on that side and check that the ignitor is in good shape. Tap/shake the cat to see if it rattles (it shouldn't). If you can (not sure how easy this is - haven't tried it myself), unbolt the cat and look down in it to see if the grid is intact (not broken, not melted/clogged).

Also check for good wiring on the sensors (that it hasn't been chewed by squirrels).

If all of that is good, seek professional assistance.

Chip H.

BlackCat81

A bad coil would cause a misfire, a bad plug would cause a misfire, wiring damage would cause a specific code related to the component it's affecting. It needs a cat.

chiph9

OP hasn't mentioned if other codes were set. And checking those is cheap and easy.

Chip H.

hopk1719

I will be checking for other codes tomorrow. If no other codes, I will be trying an o2 spacer. Just bought the vehicle 2 months ago and still need to do timing belt/water pump/ thermostat

hopk1719

Voltage at idle? Shouldn't bank 2 sensor 2 be a flatter? That is an original o2 sensor.

hopk1719

I keep seeing bank 2 sensor 2 really volt spiking. The replaced sensor seems to be operating properly.