How to fix code P1121 on 2000 Lexus gs300 ?

SOURCE: HAVE A CODE PO446 1999

P0446 is an evap code. Most likely, you need to change the charcoal canister vent valve (CCV) under the trunk floor. Fuel cap codes set 3 codes at once, P0440. P0441 and P0446, all at the same time.

SOURCE: engine stalls after cold start-up & release from high speed idle

I had a similar problem.  I found the filter cartridge below the pcv valve had carboned up, and after the crank case pressurized, oil was forced out the pcv supply line to the throttle body.  The idle air control valve solenoid got oil in it and caused my problem, but only when it was cold.  The pcv cartridge is accessed by removing the pcv valve and the rubber fitting that the pcv valve mounts in.  It can then be fished out with a pick set, and it is a pain because you can't see it, but you can get you hand back there.  I hope this helps you.

SOURCE: I have a 2001 Lexus gs300 and I need to repair

Buy the sensors and put them on yourself. Buy the aftermarket. They are the same. OEM will cost you about $110 each, Aftermarket $65 each. I work for Bosch and run the line that makes oxygen sensors, there's a lot people don't know about aftermarket and OEM. These can be removed with a 22mm wrench or you buy a special oxygen sensor removal socket/wrench. It's a matter of getting it loose. Run the engine for 1-2 min to warm the exhaust, this helps to loosen the sensors and keeps from stripping the threads. If a shop does this $250-325 depending on shop rates.

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SOURCE: 1999 gs300 shows code p1349 and the engine runs

Its a Oil Control Valve most likely. Get the valve. Its like 100 from Lexus. Mine did the exact same thing with that code. GS300

SOURCE: my 2005 altima idles high and engine code

Your Altima will require an idle air volume learn after cleaning. This procedure will set your idle speed. If the code keeps recurring after the IAVL your throttle chamber may need to be replaced.