Chevy HHR Network - New HHR owner

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My roadmaster wagon finally had to be retired and i needed a replacement that would fit my work needs and found a 2006 HHR LT1? it has the leather interior in surprisingly good shape for its age and miles. Besides being filthy when i bought it at a used lot, the biggest annoyance so far is the P0420 code coming up a lot along with the intermittent rough idle while in gear (but interestingly, not if put into neutral). Hope i can help contribute on this forum like i have on others.

Hi Retro, :welcomewagon: to the forums!

Welcome to the site, the P0420 could be several things,

https://www.obd-codes.com/p0420

look at the easy things first, sometimes they are easy and simple fix.

yes - i've had the loose gas cap message a few times too - might need a new one based on how the seal looks

Save yourself a bunch of agony and just replace the catalytic converter with a good quality one. Yes, P0420 could be something else, but pretty darn rarely. Strange that is the ONLY code when it is "rough idling". You probably have horrible gas mileage.

i read often could be o2 sensor. Rough idling might not be best description - more of idles around 800 and then occassionally will drop 20-50rpm for a split second causing that 'roughness'. MPG is pretty good - about 30-32 on hwy and 25-26 town

thanks for the links to other posts - as this car was already at 169K when i bought it i don't know the history. Right away i had to replace the lower arms due to shot bushings before i could get it aligned. ran a can of seafoam thru the gas tank and drove across florida back and forth - about 160 miles each way. on the way back a few weeks ago, car was running pretty crappy until i had been driving on the freeway for about 15-30 min... had the scanner monitoring the RPMs the whole way... After that first 30min on the way home, the car ran smooth as can be for a 4banger. since then a few hundred miles of mostly around tampa bay area - lots of stop and go. Never has been as bad since, so thinking the injectors were prob a little dirty too.
When i do an oil change soon I'm going to check and clean the connections on the O2 sensors.

Oh, and on another forum where certain cars have issues with the cats after 20 years, i read someone who recommended removing the upline o2 sensor and spraying the seafoam spray version up in there ahead of the cats and then taking a robust highway drive to 'clean' out the cats. Not sure that's a smart idea or effective.

I’ve never seen a catalytic converter that reacted well to any cleaners, they just failed, there are many videos in the internet pertaining to that, but it doesn’t work out well .

P0420 is triggered by the #2 O2 sensor.. I spent a lot of money and time trying to avoid getting a new cat ending with having the car repossessed.

Just get on ebay and get a new downpipe/cat. That will solve 2 problems; theP0420 and the imminent flex pipe for less than $200. Not a bad idea to replace both sensors at that time.

However, you need to figure out what caused the failure or it will happen again. Excessive oil consumption is at the top, as is either rich or lean A/F mixture, misfires, silicone and other elements getting into the combustion etc.

But, go ahead disbelieve.