Brake Lights, Pontiac Thermostat, GMC P0420 Catalytic Converter

Question: 2008 jeep less ths 40 K miles. Well taken care of. Brake light comes on after driving about 12 blocks then goes off.
 Mechanic: Is it the red brake light?
Yes. Goes off after about 20 scs.?
 OK. First thing to do is check the brake fluid level. If it is low, the light will come on. After driving and the fluid sloshes around a little, it can then go off.
Had fluid checked and it was full. I am wondering if it might be the cold weather snap.
 Check your emergency brake. Make sure it is not partially engaged.

Yep checked that also.
 Hmm. Strange. Maybe a faulty fluid level switch then in the reservoir.
Thanks. I am female so thanks for the help. Time to find a jeep dealer in my area.
 You can take it to any local shop if you want.. The dealer is quite expensive.
Thank you.

Question: `1997 pontiac sunfire thermostat location? I drive my car a short distance and it gets hot was going to start off with the thermostat.
 Mechanic: What engine size?
2.2L
 One moment... The upper radiator on the drivers side under the hood, follow it from the radiator toward the engine. It is in the housing the hose connects to.

On the top?
 Yes.
Does it have three screws or two?
 It has two.
Lower hose or upper coming out of water pump?
 Upper i believe. Trying to remember back 16 years
Cuz he took off and there was no thermostat in there.
 HMM. Try the other hose then. They moved them around in different years. Also, if you just got the car, maybe they took it out to try to stop an overheating problem.

What would suggest if car was getting hot driving short distance in town ? Start with thermostat? It just started doing this two days ago and I've had the car 18mo and no problems till now. Culd it be the water pump ?
 Start with replaceing the thermostat.

Question: Changed both o2 sensors catalytic converter, repaired exhaust leak and replaced stuck thermostat on 2007 GMC envoy and still getting code p0420 code with no other codes.
 Mechanic: Has the truck been driven a good while after the catalytic converter replacement?
Over 600 miles. Code has been reset twice.
 Hmm. What kind of Cat. was it? Brand, cost.
Walker direct replacement little over 400 bucks. My mechanic seems to think I may have gotten a bad one he says it doesn't happen often but it does happen.
 Yes, that is why i asked. Also, the cheap catalytic convterters can also set that code because they are not efficient enough.
He said the same thing to me when I inquired about a Weld in aftermarket. It is showing no restriction when he does a pressure test on it.
 Yes, they do not show restriction, t`hey are just not efficient enough. Hard to diagnose.

We are planning on doing a pressure test on the fuel system as well this week I just wonder if anybody else has any other ideas.
 Pretty straight forward code. Something not right with the converter i think.
He has been working on cars for 30 years and is at a loss. He doesnt think its a fuel issue because he said I should get a code for to rich or too lean or a misfire
 Let me check something... There is possibly a reflash of the computer software for this condition. Only way to check is to call your local dealer, give them the VIN # and they will run a check on that.
Alright thanks for the information hopefully it doesn't cost me a million dollars lol.