We only drive it on the weekends to the local farmers Market …maybe 30 miles round trip on the weekends…just randomly threw the code.I have a OBD2 scanner which suggested the downstream 02 sensor or the cat.conv needs replacing.Are the Bosch sensors just junk out of the box ? I’m going to hopefully pull it apart this weekend if the cat.conv arrives on time.

 

Im assuming that’s a dealer only job ?

 

Won’t affect many HHR’s but wow, the whole thing is pretty interesting, not just the part about P0420.

“DTC P0420 Catalyst System Low Efficiency - The ECM has determined the catalyst efficiency has degraded below a calibrated threshold. The ECM may prematurely diagnose a catalyst failure before the catalyst has been allowed to run hot enough for a sufficient amount of time to burn off manufacturing coatings inside the converter. These coatings adversely affect the catalyst efficiency and the O2 sensor performance.”

Pretty sure “manufacturing coatings” ain’t the problem in this case though, 17 years later, lol.

 

If you have a correct VIN it might be free.. That is the last 8 digits.
Bosch sensors have a history of being junk. NGK/NTK makes the ACDelco/GM part.

 

The TSB doesn’t mean the parts and labour are free, just a Technical Service Bulletin, to help the mechanic figure out what’s wrong

 

Do you have a public road permit? If yes, then no problem. Even if the exhaust is bad.
Do not take my words as a guide to action in US (I don’t understand your laws, maybe it’s punishable), but if it was my car in my country:
1. visit to a small auto repair shop, diagnostics with a professional scanner, detection of stored errors on the catalyst;
2. destruction in the on-board computer of all traces indicating poor exhaust or improper operation of the vehicle that killed the catalyst;
3. visit to a car dealer, initiation of the procedure for warranty replacement of the catalyst.

 

If Your state does not require emissions inspection, it does not matter if the cat is good, bad or not installed. (Public road permit)
The federal warranty is 8 years/80,000 miles. Way too late.
The freeze frame data might be of some use figuring out the why it failed.

Codes don't "randomly" set. They set for a reason; the test failed.

Any shop that has access to GM Techline can perform firmware updates.