I did notice a drop of about 1mpg. Now that it is fixed my mpg is back up to the 22mpg range (was 19) with the same driving conditions.

 

If the cat goes bad and restricts the exhaust, your milage would decrease. Good tips here on what to look for, thanks folks!

Jim

 

I'm having the same issue noise under acceleration only, have a manual and it is the same when I am in reverse or gears 1-5 especially 5th. When cruising can't hear it until about 3000. I tried to look under but to low in the front for me to see the cat I can only see the pipe from the cat out, I tried to look from the hood but I have the stock box and can't really see all that good back there, and with the exhaust I can't really tell outside. I do know that in side of the vehicle when driving and the windows rolled up, stereo off the noise seems to be coming from the center console just around the shifter but just under the ac control unit. With the hood up you can't hear it either. I already made the appointment for tomorrow with the dealer so we'll see. Maybe just a coincidence BullyWagon but I was at 39K and just hit 40K not too long ago. I too had to have the BCM replaced,a new thermostat the original was stuck open and a new sunroof. My heat shield also cracked at 15K miles.

 

That sounds like the cat for sure STR8... I had the thermostat,and BCM X2... no sunroof problems though.

The best way to check the cat is to look under the car at night... Mine was glowing after 45min at freeway speeds.

 

It's been a long time since my last reply, my PC crashed. Since then the HHR has suffered some body damage. When we had the real bad winds and fires here in So Cal in October. But she's all good in that aspect. As for the noise, worse then ever. Last Friday night driving home, just happen to turn the music down on the Freeway because it sounded like a major exhaust leak just under the gear box. So I gunned it for a min and egg smell. I've been around enough poorly tuned cars to know the smell. So took her in today. Cat is done. They said they put a few holes in it to help it breath better, but the internally it is finished, which explains the slight rattle I would hear in the morning when cold. So like everything else on the HHR, they have to special order it, even though it is the same cat on the HHR 2.4, Malibu 2.4, and the Cobalt 2.4. Not to mention the Pontiac G5 and the Saturn Aura, my dealership sales all 3 brands and no parts in stock. So Friday, Monday service is closed for Christmas eve, and of course Christmas day, so May be Wednesday. So I gotta drive her the rest of the week like that, it's all good though at least they finally figured it out. The first time I took it in they said it is supposed to sound that way. There was no way this time the tech could say that. Note just as Bully Wagon said, the Cat was glowing red, and my millage dropped to about 24 Hwy. No power loss at all, she revs better then ever.

 

That's strange about your cat. converter. I would think that SPO in SoCal would have those stocked for immediate shipment to dealers.

But, the question is.....what made the cat go bad prematurely. Is your car running rich/lean? Good thing the cat is part of the emission system. It should be covered by that warranty.