Housing under the manifold and the converter

along the L and R side of the block under exhaust manifolds

follow the top radiator hose to the thermostat housing i believe it is held on by three bolts

3.4L Engine

1. Remove the thermostat housing to intake manifold bolts.
   
2. Remove the thermostat housing outlet and thermostat

3.1L Engine

1. Remove thermostat housing-to-intake manifold attaching bolt and nut, then remove the housing.
   
2. Remove the thermostat.

2.2L Engine

1. Remove the thermostat housing to water pump feed pipe bolts.
   
2. Remove the thermostat housing to water pump feed pipe.
   
3. Remove the thermostat.

Hope this helps; also keep in mind that your feedback is important and I'll appreciate your time and consideration if you leave some testimonial comment about this answer.

Thank you for using Fixya, and have a nice day.

LOOK UNDER THE EXHAUST MANIFOLD HEAT SHIELD.YOU WILL SEE THERMOSTAT HOUSING WITH THE COOLANT INLET PIPE CONNECTED TO IT.

LOOK UNDER THE EXHAUST MANIFOLD HEAT SHIELD.THERMOSTAT HOUSING HAVE COOLANT INLET PIPE CONNECTED TO IT.PAY ATTENTION TO DIRECTION OF THEMOSTAT SO YOU CAN INSTALL IT BACK IN SAME DIRECTION.

The thermostat is virtually always right at the engine end of the upper radiator hose. There is virtually always a housing that the hose clamps onto, which housing is then bolted onto the engine. The thermostat is under that housing.

THE 2.4 ENGINE THERMOSTAT LOCATED NEAR THE EXHAUST MANIFOLD YOU HAVE TO REMOVE MANIFOLD HEAT SHIELD TO GET TO IT. THE 3.4 AND 3.1 THERMOSTAT UNDER LOWER INTAKE MANIFOLD ON DRIVER SIDE.YOU HAVE TO REMOVE THROTTLE BODY UNIT AND COOLANT LINES HOOK TO IT TO MAKE EASIER TO REMOVE THERMOSTAT HOUSING TO REPLACE THERMOSTAT.

I had the same problem. My cat. converter went which plugged my muffler and choked out the car seconds after starting it. I took the exhaust system apart between the manifold and and cat. (3 bolts behind the motor by the firewall) then started the car and ran fine. I now need a new cat. and muffler for my 2.2 Alero.

Hello,
The O2 sensor is in the exhaust. There also may be two of them. Here are the most common places for the O2 sensors. One after the intake manifold before the catalytic converter, and then there should be one after the catalytic converter in the exhaust pipe. You will see them, they have wires that run to the back of the motor. You can just simply unplug the wires and unbolt them to replace them.
Hope this helps! -Steve

check the housing onder the manifold and the converter they might be clugged up and keeping it from starting right away.