Hello everyone. First I'd like to start by thanking Jerry Suarez & his crew down at Discount Tires in Azusa, CA and Gabriel Alcala, a diagnostic mechanic of 35 yrs experience, at A-1 Auto Expert in Chino, CA. Just as important as fixing your vehicle is finding a mechanic who is willing to listen. I hope this helps those of you out there still searching for a solution. TLDR below story

My Story:
Check engine light came on May 16, 2022 and the device I have attached to my ODBII told me it was the dreaded P0420. My brain went the same places everyone else's have, new catalytic converter(twice), new O²'s and it still popped again & again.

Jerry & crew discovered 2 of the issues. First an overcharging alternator, which after 3 months in & out of both shops we discovered is the keystone to this whole issue and second a crack at the wastegate in the exhaust manifold (see pics below)

While the alternator is seemingly irrelevant to a P0420, after switching shops in July (as Jerry & crew only do mechanical) Gabriel discovered that the overcharge to the system caused serious damage to the ECM, caused a short in the wiring harness leading to the downstream O² sensor causing misreads and also blew out several fuses mostly related to accessories.

I believe GM had to be aware of this issue of their stock alternators being prone to overcharge given the sheer number of P0420 posts I've seen. I'm certain the fact that it does not directly affect the safety of the vehicle, is why no recall was issued.

Had the ECM replaced and reprogrammed (which is specific to your vehicle, can't be straight swapped), replaced the damaged sensors, wiring & fuses. After everything was electrically and mechanically right with the car I had to go back and have the cat replaced a 3rd time because the 2nd replacement an aftermarket unit had been installed after the 1st replacement(an OEM unit) failed to clear the light.

Luckily both replacements were still under warranty of the 1st replacement as CARB approved cats are $1000+. I cannot stress this enough, especially if you're in California like myself. You cannot get that godforsaken light off or pass SMOG with a 1.4L unless you use an OEM Catalytic converter direct from a GM dealership. I've heard you can get away with it if you have the 1.8L but not in the 1.4L.

If you made it to the end of my story now you know what to look for. Find a mechanic who'll listen or if you wanna have the fun yourself good luck to you. I know I could've shortened it but I wanted to do my car, my wallet, the 3 months of my time and most importantly everyone who has dealt with this issue justice. Thank you for reading


TLDR:
This issue is mechanical, electrical and computer related in nature. Alternators installed on 1st GEN Cruze are prone to overcharge, leading to appearance of code P0420. I believe GM is aware of this and should've issued a recall.

Overcharge causes damage to ECM, shorts in wiring harness leading to O² sensors, burn out fuses as well as potentially damaging other electrical systems. Find a mechanic who will listen or if you have the know how have fun.

Check voltage output but keep in mind with alternators the problem may be intermittent. May also cause physical damage due to ECM malfunctioning such as cracks developing in exhaust manifold (see pics)


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Thank you!! I'm running on avg.14.9 to 15.1 volts. I suspect that's too high or overcharging??

 

AGM batteries are designed for a MAXIMUM charging voltage of 2.4 volts per cell (14.4v for 12v batt). GM warns not to exceed 14.6 Volts. You can get away with 14.9v with an EFB batt.

 

The charging system on this car is very sophisticated. Driving down the road you may see a voltage of 12.5-14.9 and there be absolutely nothing wrong. The ECM, uses several inputs (battery temp, amp clamp amperage, outside and engine bay temp). It then looks at the battery voltage and demand and chooses 1 of 5 different strategies. Service info is needed to work on this charging system. It is all based on network messaging between the Alternator, ecm, bcm and the instrument cluster.
Believe me , the first time I drove one monitoring voltage on the IC I was intrigued at the changing voltages.
If you are going to work on any of these cars today you must understand how the system was designed to work. I have been doing this for 35 years, every time I say "well thats just a battery change, just change it", well my friends that day that BMW was a drive in and push out. Alldata has a diy section that will give you all of the info I use for about 30.00 per car and it is for 3-5 years (not positive). GM will give you the service info their techs use cheap too. I have a friend in oklahoma I call for help, his first words are "DID YOU READ SERVICE INFO".