P0300, P0420, P0430 Codes: Bad Catalytic Converters

This post chronicles a major weakness in the Frontier design that I uncovered (and solved) that concerns the catalytic converter system. I have the 4.0 L, which has a primary catalytic converter and a secondary catalytic converter on each cylinder bank for a total of four converters. I will refer to these units as “cat(s).” For most cars, catalytic converters are designed to last the life of the car. From my experience, this is not true for the Frontier. Scotty Kilmer, has a low opinion of Nissan quality and has specifically identified Nissan cats as being poor quality. :frown:

In July 2018, my mileage went over 105k which triggered the need to replace the spark plugs. I chose to use Iridium NGKs rather than Platinum. After the plugs were changed out, the engine almost immediately threw bad catalytic converter codes: P0420 and P0430. The engine ran fine and it seemed nonsense that new spark plugs could damage the cats: I considered the codes to be a “false positive.” I continued to drive the truck for if the cats were truly dead, I could not do any further damage. All along, I was struggling to figure out how new plugs killed the cats. If you check the ClubFrontier forums, you will find other incidences of new spark plugs killing the cats. Very strange.

In October, my Nismo started to throw P0300 codes (random engine misfire). These were flashing which indicates a serious error. It was then the cats were examined using SCAN on the upstream and downstream O2 sensors. Yep, the cats were dead. The tech said if they were changed out, it would also stop the engine misfire P0300 code. With the forebearance of $1,500:crying:, the cats were changed out along with the two upstream O2 sensors. It is axiomatic that if one makes a large donation at the car repair altar, good things will happen. The dead cat codes, P0420 and P0430 no longer registered. The engine ran very well, idled smoothly and could rev to 6k rpm. But the random misfire code, P0300 remained. In fact, it appeared much more often and continued to flash much longer. I could no longer drive the Nismo because the misfires could send excess hydrocarbons into the exhaust, which will damage my new cats.

The tech then utilized a rigorous protocol to uncover the reason for the misfires. All ignition electronics including the new plugs were checked. Fuel pressure and volume flow were checked. The intake plenum was checked with carburetor cleaner and smoked to find small leaks. MAF and EGR were check out. All injectors were tested and cleaned as needed. Compression and valve timing were checked. Nothing worked as the truck continue to throw P0300 codes in a fast and furious pace. The tech spent a lot of hours on the truck and had nothing to show for his efforts. He personally did not think the engine was misfiring.

We were running out of options. My research in this Clubfrontier website came up with a post of another owner who also had P0300 woes. His solution was to gut the secondary cats and the problem went away. I gave this tip to my tech and he ran with it. In his tests with disconnected secondary cats (very loud!) he did not get the P0300 code. This was the smoking gun, the secondary cats were plugged!:surprise: Another benefit discovered during this test is that my engine put out significantly more power (10-20%). The secondary cats were replaced, no more codes. Problem solved. I can now drive my Nismo. Thanks to this Clubfrontier Board, I was able to alert my tech on the plugged secondary cat root cause.


Analysis:

How did changing of the spark plugs cause the P0420 and P0430 codes to appear? Best guess: New spark plugs will provide a better spark. How this played out in the exhaust and upstream O2 sensors probably caused the engine control module to recognize dead cats.

How can an engine rev to 6K rpm with plugged secondary cats? The secondaries were only partially plugged and exerted backpressure on the exhaust that in turn affected the state of the cylinder contents during induction, which in turn affected the stoichiometry. This was enough to signal a misfire. The misfire was not due to the crankshaft sensor data feed but the O2 data feed from the upstream O2 sensor. Most techs will not diagnose any plugged exhaust components if the engine can rev freely. In other words, they way the secondaries failed did not provide enough evidence to a tech to quickly diagnose the issue. Major issue.

Lessons learned:

I needed to change my clutch last year. If I knew then about the weakness in Nissan cats, I would have changed them out then and save a lot of labor cost. Cats need to be taken out to change the clutch.

Nissan warrants the cats for 8 years/80k miles. Knowing what I know now, I would have changed spark plugs before 8/80 to see if the cats died. If so, the cats would be replaced under warranty.

It is troubling that partially plugged secondary cats can rob an engine of so much power. The loss is so subtle and slow that I did not recognize it. If you think your 4.0 has lost a lot of mojo, break the primary/secondary cat flange, run the truck, and see if the lost power reappears.