Weird, inconsistant P0420 situation. Fluke? Bad gas? Elevation?

This will be a long post, but I hope to tap into the wealth of knowledge here, and gain some insight from anyone who has experienced this problem. I searched and found a lot of good info and plan to run some tests to narrow down the potential cause. This is a great link posted by another user:
https://parts.olathetoyota.com/p0420-code-toyota

I just completed a 2,453 mile road trip in my 2000 SR5 V6 4X4. The truck ran FLAWLESSLY with only a slight, inconsistent stumble when cranking the engine on hot start. My suspicion is that it relates to a failing oxygen or A/F ratio sensor on Bank 1, or a bad catalytic converter (hopefully not).

Pre-trip info to provide a background:
Purchased in CA with 186,799 miles, 3/12/22. Passed smog inspection on 1/13/22. Some deferred maintenance allowed me to negotiate a significant amount off the asking price. Very clean, well taken care of, all original truck.

Photos to keep the thread interesting. Morning after I bought it and camping the following weekend





I completed a minor service @ 187,215: oil/filter, air filter, fuel filter, new gas cap and Truestart 24F battery.

MAJOR service completed @ 188,500 miles with only Genuine Toyota parts. The mechanic is a shop owner who works on all types of Toyotas regularly. He is a good friend and a Toyota enthusiast who owns a 1st Gen Tacoma here, a Hilux Diesel in Honduras, and a few other Toyotas in his family.

Parts replaced:

• Radiator, upper and lower hoses, thermostat, water pump
  
• Timing belt, tensioner, idlers
  
• Plugs, wires, coil packs
• Rebuilt, OE injectors from CS Performance, matched to 1% flow rate
  
• PCV, valve cover gaskets, plenum gaskets
• Cam seals, crank seal, rear cam plugs and various other washers and gaskets
• Trans fluid drain and fill (filter cleaned, will be replaced @ next interval)

1,500 miles driven after the major service and the truck had absolutely ZERO issues, ran like new. The Check Engine light illuminated at about 190,000 miles after filling with questionable gas in Bishop, CA. Oil and filter replaced by a shop in AZ @190,154. Fault code was read then.

The Check engine light illuminated multiple times during the trip. I reset it manually by disconnecting the battery for 30 minutes, the shop reset it with equipment. Each time it returned after about 250 miles of driving, sometimes sooner.
I reset the CEL on the morning of 5/20/22 and it remains off on 5/22/22. The 638 miles of driving on the last two days of the trip involved high and low elevation, mostly cruise control highway miles, and alternation between Regular 89 and Premium 91. Average of 22.01 MPG over 2,453 miles, with the best tank yielding 25 MPG.

If you made it this far, thanks for reading!

So, after that long winded post, what are people's thoughts on the situation? To recap: Inconsistent P0420 code. Truck runs and cold starts perfectly, only issue is a slight, inconsistent stumble on hot start.

 

I drive from Charlotte NC to Plymouth MA and back every year.

50% of the time it trips on one of the legs.

I reset it. May come back once or twice and then stays off.

Have been doing it for years now.

I can go all year and never see that code with my normal commutes.

These engines are notorious to trip that code during long treks where you are just cruising at 60-80 for long stretches.

-Mike

 

Strange. Any thoughts or theories on the cause? I'm guessing that this did not happen when these vehicles were new.

When cold, the engine revolves 6 times before firing every time. When hot, it varies between 1 and 3 revolutions.

How many miles on your truck, Mike? How many revolutions of the engine when cold and hot starting?

 

I ahve foudn that a lot fo rebuilt injectors are not consistent. Bad seal, intermitent issue with the diode, poor spray pattern that is consistent with the fule issue you mentioned. Have you run an injector cleaner through it? Try this:

Lucas FI cleaner in a fresh tank of 89 or above.
Run a fresh tank of 86 from anywhere but Walmart, etc.
Rerun FI cleaner with a fresh tank of 86 from anywhere but Walmart, etc.

Its a cheap easy place to start eliminating before going down the sensor highway.

 

Thanks for the reply, but I doubt it's the injectors. They are fresh off a blueprinted rebuild, flow matched within 1% of each other, and the truck has no symptoms of a misfire. I put 1500 miles on the truck after the injectors were installed during major service, and it ran like a top.

Agreed. It is a cheap place to start, so I will try some fuel additive for the heck of it, but sometimes I feel like they are selling snake oil.

 

My codes started around 100k miles on long trips. Rig has 270k miles and only on its second A/F sensor. It passes smog every year.

I've owned the car since new (2002).

It's a dirty system. The CAT will lose a little efficiency no matter what. The A/F sensor will wear out. The MAF gets dirty, etc etc. With 20 years of reliability, I'll live with the occasional CEL resets.

 

My 2002 with 203K has had the P0420 come on in December 2021. I reset it a couple of times before replacing the A/F & O2 sensors with Denso units.

It was good for about 800+ miles before the code reappeared. Sometimes the “CEL” would not appear during drives but it still shows “PENDING” on the logs. For example: The CEL would not appear after startup after a long freeway drive. Then it would appear after a city drive. Weird! So my guess was one of the catalytics was going bad.

I replaced the typical gaskets, seals, timing belt, water pump, injectors, exhaust gaskets, etc about 7K miles ago.

I thought about purchasing replacement catalytics but wanted to know which unit was causing the issue. Luckily I was able to secure OEM units from a friend to test my theory.

I replaced the front unit only. So far it’s been about 900+ miles and the light has not come back. I will return my stock unit in 100 miles and replace the back unit to see if that causes a code.

 

Thank for your input; it sounds almost identical to my scenario. I will run some tests and, if necessary, replace the downstream O2 sensor. The upstream A/F ratio sensor looks to have been changed recently by the previous owner. If that doesn't solve the problem the cats will be next on the list. Luckily I get some heavy discounts from Toyota dealership connections.

 

Even with heavy discounts, those OEM catalytics will be expensive. Some have suggested Magnaflow or Walkers as good replacements.

 

I'm learning a lot from this thread. Good stuff!

Thanks for listing other options. It can be difficult to find aftermarket cats that have the CARB executive order number, any vehicle will immediately fail a visual test without that. However it looks like Walker has us vintage Toyota nuts covered, very good to know.

Vehicles are my hobby, so I almost always fork over the extra cash for the Genuine parts. More often than not, I spend less for Genuine then I would for the same aftermarket part. My philosophy is work hard, set money aside, and buy the best. Can't take it with you...