Ohirolina

Engine light came on today. Went to Auto Zone and had them check it. Their machine read a PO420 code Catalyst System Low efficiency (Bank 1) They guy said that was all he could do. He didn't know what it meant exactly.
I was wondering if it couldn't be a 02 sensor since I replaced the first exhaust pipe coming off of the manifold last weekend and it had 2 catalytic converters and two 02 sensors.
2010 HHR LS Panel 2.2 L 73,000 miles

Oldblue

Check the O2 sensor connections, and the gasket at the exhaust manifold.

donbrew

It "could" be an O2 sensor or a leaky gasket. But, sad to say, most likely you need a new Cat. It means that the #2 O2 sensor is working and finding something wrong. "Could" be the sensor is not working correctly or something else is causing the failed test.

I went through much the same with my 2011, I never did find the problem. Did you ignore the CEL for a long time?

Ohirolina

The light just came on today. Can the Catalytic Converters be bad from the factory? The flex pipe was cracked on the pipe I replaced. There was no light on before I replaced it.

donbrew

What exactly did you replace? The flex section, the entire downpipe with the cats.

Ohirolina

The entire downpipe with both cats

donbrew

Yes the cat can be defective. Especially if not GM branded.
There are warnings all over the place about after market cats. They frequently have bad weldments that cause leaks, they rarely carry the same load of elements etc.

There is this tidbit:

Ohirolina

Hopefully it being new is the reason. Probably only has about 75 miles on it. I'll run it a few more times then get the code cleared and see if that helps.
Getting ready to check out the 02 sensor connections in a few minutes when I get back to swapping out the motor an transmission mounts.

donbrew

The code will clear itself as soon as the problem is resolved. Go for a long highway trip, note it says an HOUR AT HIGHWAY SPEEDS.

Ohirolina

Thanks. Of the 75 miles, only about thirty were at highway speed. MERRY CHRISTMAS!!!