Error Codes reads '1221'

[Calico_NC]

I know its an O2 senor, but i just wanna make sure b4 i replace it. Tell me if these symptoms are correct if u have a bad O2 sensor:

1) When I start up the car, it shakes vigorously when in idle.

2) Doesn't feel quite right when I'm driving. Kinda like a constant rumble or hesitation.

Couple Question:
Do you think it's my sensor? If so, is it a Do-It-Yourself job, or do i have to go through a dealer/shop?

And...what will happen if it isn't replaced immediately?

[shogun]

Check first all the other stuff like air leaks etc.
The sensor might be o.k., but it gets wrong signals because something in the intake is making a problem.
If you start a cold engine, the sensor is not working till the cat is heated up to I think 600 degree C. MOTRONIC works at that time with a fixed value.

[stilljester]

How long has it been doing this? Are you sure its running on all cylinders?

[Calico_NC]

I have seen a check engine light in the past, but it went away, so i didn't get a chance to check it. but its been having a rough idle for a while now. I would say on and off for a couple of months (maybe 4 or 5). Don't know about the cylinder thing, but my guess is yeah, it's on all cylinders. But i know for sure that something is wrong because it has never been this bad before.

[632 Regal]

you should check the wires/cap/rotor and plugs before the sensor. the give away is that its missing when you just start it and like Shogun stated the sensor isnt working at that time.

[Bill R.]

have wires/cap/rotor/ his is distributorless.... Coil on plugs. He's probably got either a bad plug, or he may have a coil shorting to ground from a leaking valve cover gasket filling the spark plug tube with oil which may have fried the coil by now and or fried the DME as well....Other possible causes are an airleak after the MAF causing it to run excessively lean at idle. etc etc.

you should check the wires/cap/rotor and plugs before the sensor. the give away is that its missing when you just start it and like Shogun stated the sensor isnt working at that time.