I've looked for a while trying to find a post like this, and its probably out there. But anyways;

First my specs 90' Pick up, 4x4 reg cab, 22RE, 5 speed standard.
When i started my truck had a blown head gasket and NO check engine light came on.
I tore it down and replaced the head gasket myself, and figured while i was at it lapped the valve seats and put in all new valves, i also replaced every gasket along the way when putting it back together.
I started it up to adjust the timing, got everything set (a little on the leading side)
And went for a test drive, when i revved it up to just before shifting from 2nd to 3rd the check engine light came on.
I went home, used the jumper trick and got error code 21 (O2 sensor).
I drove it for the next week and the same thing would happen, i'd start of driving with no lights, and after i revved it (didn't matter what gear i was in, even neutral) and the light would come on, same code.
So i replaced the O2 sensor. 75 bucks i didn't want to spend but oh well.
So new sensor installed went for a drive, and sure enough rev it up and light comes back on!!
So i checked the connector for the O2 sensor, cleaned it up as there was a good deal of dirt/oil/grime on them, and plugged it in, same thing, rev the engine light comes on.
Also now its giving me 21 & 42 (variable speed selector), i don't even know what that is, let alone how to fix it!
Anyone have any ideas as to whats wrong with my little yota?
I've checked all the bolts in the exhaust system, and the manifold to the block only has about 5 of the 8 bolts so there could be a small leak there, I heated up and torqued the header to the down pipe connection.
Any help would be appreciated!

 

What brand 02 sensor did you buy ?, if its a Bosch 02 sensor they don`t work well in toyota engines. You need to use a Denso 02 sensor, that1s an OEM replacement.

 

It was in fact a Bosch, I figured that there was an exhaust leak somewhere and it was giving me a false code.
My little truck is in the shop this week for a heart transplant no more burning through tons of oil for me! for a while anyways...
Had a wrist pin tapping and rather than deal with it tore apart in my garage for who knows how long i bit the bullet and took it to a shop.
When it gets back I will look into another O2 sensor.
Thank you for your help!

 

yeah I went through 2 Bosch's before I got a Denso

both of the Bosch sensors were throwing the CEL on, despite being brand new

installed the Denso, and problem solved

 

I ordered a denso off of amazon, (only took like 2 days from order to doorstep) and installed it yesterday afternoon, i started up the truck and the cel was off, but as soon as i backed out of the driveway and took off it came back on, so i pulled back in, read the code (still the O2 sensor) i disconnected the battery for about 45 minutes while i had dinner, came back out, hooked it up and went for a drive!
CEL still on
So i checked online and saw someone said to pull the EFI fuse, turn the key to ON and wait 1-2 minutes, turn to OFF, reinstall and try again.
CEL still on
Is there a possibility that the connection that plugs into the O2 sensor is bad?
Does anyone know where the other end of the wiring is? maybe I knocked something loose?

 

Check the wiring. With the key on, one terminal for the o2 sensor should have power. The other three wires should have continuity to the ecu. Look in the FSM for the terminal pinouts to check.