The good ole' money light came on in my 99 Suburban 5.7 with a P0420. Cold starts are fine, no loss in power, running smooth as silk and I am still getting about 450 miles on a tank of gas (42 Gal tank). I replaced both cats last year too. I just don't know what could be setting this code. Could it be the O2 sensor(s), but if it was, you would think the gas mileage would be down. She just rolled over 200K this morning.

Thanks,
Cole L.

 

Oxygen sensors don’t have a direct effect on fuel mileage - even running in open loop constantly, fuel mileage wouldn’t suffer so bad that you’d notice it.

Additionally, P0420 is specific to the efficiency of the converter, not the engine, and is set based on downstream (post converter) sensor input. The downstream sensor is only a monitor for the performance of the converter and has no effect on engine operation whatsoever.

Did the code start setting before or after the new converters?

If the engine is running smoothly (no misfires or reason to suspect leaking injectors), possible causes are:

-air intake not properly sealed, allowing dirt/dust into engine which can then travel through the engine and damage the converter (GM issued a bulletin regarding this on some of their trucks built in about the past 5 years or so - I know it was an issue for the 2015 models - so this might be worth investigating even though yours is a 99)

-exhaust leak anywhere between the cylinder head and downstream sensor of the affected bank (depending on its size and location, a leak may not be loud enough to hear and will only be found using smoke testing)

-incorrect or faulty sensor or sensor wiring

-failing converter - even though it’s new doesn’t mean it’s good. A problem not caught during manufacturing, shipping damage, or improper installation could have affected its performance.

-incorrect converter for the application

-PCM failure

-driving habits such as frequent, hard acceleration, short trips, off-roading, and towing

Converters don’t go bad for no reason. Age and mileage mean nothing - as long as the exhaust gases that enter the converter are within its operable limits (in other words, there’s not a bunch of unburned fuel getting dumped into the exhaust), the converter will work.

 

I test drive and monitor the pre cat and post cat 02 sensor switching rates. the pcm is comparing the cross counts of the rear and front 02. when the rear switch fast the cat isn't storing oxygen. sometimes new rear 02 and old front 02 can cause false efficiency dtcs. as long as the 02 are switching properly... the cat is nfg. Don't buy a cheap cat...they have low cerium content...the oxygen storing element....p0420 p0430 will reset with a cheap cat.