so, i've had the cel on for like 6 months with code 21, even hooked it up to a datalogger to be sure and finally i replaced the rear 02 sensor, bought it from the dealership for $160. i turned the car back on and the cel is still on. unplugged the battery, it went away, but came back with the same code a couple days later..wtf? could the code 21 mean something else besides the rear 02 sensor?

21
Upstream HO2S stays at center
Harness and connector / Upstream HO2S
21
Upstream HO2S shorted to voltage
Harness and connector / Upstream HO2S
21
Upstream HO2S response (M/T)
Harness and connector / Upstream HO2S
21
Upstream HO2S heater failure (M/T)
Harness and connector / Upstream HO2S
21
Downstream HO2S stays at center (M/T)
Harness and connector / Downstream HO2S / Fuel system
21
Downstream HO2S heater failure (M/T)
Harness and connector / Downstream HO2S
21
Downstream HO2S shorted to voltage (M/T)
Harness and connector / Downstream HO2S




current status unknown

http://hometown.aol.com/ohshootuhh/

Perf:
KYB AGX Struts
Neuspeed Sport Springs
5 Strut Bars (2 F/U, F/L, R/U, R/L)
Sprint Front Camber Kit
$15 Rear Camber Kit
Corbin's A/F Gauge
MSD DIS-2 Ignition System
Some Regrind Cams that need High Compression (which I don't have)
Ported Intake Manifold
Ported Throttle Body
Nameless Cold Air Intake with a K&N Filter
Custom 2 1/2 inch exhaust with Dual DTM Muffler
Empty Catalytic Convertor
Energy Suspension Motor Mount inserts
Hurricane_GS Short Shifter
Symborsky Shift Kit
Clutchmasters Stage 3 Modular
225/50/16inch Wheels and Tires


Audio:
Alpine CD Head Unit
Alpine Flex4 25W X 4 for Fronts and Rears
Alpine V-12 450W X 1 for Subs
Infinity Reference Series 5 1/4 and 1 inch tweeter component systems for fronts and rears
Blaupunkt 3 way 6x9's in the rears
3 MTX 5000 10" 12 ohm subs running 4 ohms mono


current status unknown

http://hometown.aol.com/ohshootuhh/

Perf:
KYB AGX Struts
Neuspeed Sport Springs
5 Strut Bars (2 F/U, F/L, R/U, R/L)
Sprint Front Camber Kit
$15 Rear Camber Kit
Corbin's A/F Gauge
MSD DIS-2 Ignition System
Some Regrind Cams that need High Compression (which I don't have)
Ported Intake Manifold
Ported Throttle Body
Nameless Cold Air Intake with a K&N Filter
Custom 2 1/2 inch exhaust with Dual DTM Muffler
Empty Catalytic Convertor
Energy Suspension Motor Mount inserts
Hurricane_GS Short Shifter
Symborsky Shift Kit
Clutchmasters Stage 3 Modular
225/50/16inch Wheels and Tires


Audio:
Alpine CD Head Unit
Alpine Flex4 25W X 4 for Fronts and Rears
Alpine V-12 450W X 1 for Subs
Infinity Reference Series 5 1/4 and 1 inch tweeter component systems for fronts and rears
Blaupunkt 3 way 6x9's in the rears
3 MTX 5000 10" 12 ohm subs running 4 ohms mono

So, yeah. Code 21 can be more than your rear O2 sensor.

Check your front.

yeah i have a code 64 and that is the rear o2 as well