P0420 help

[juice90]

Ok, so earlier in the week I posted my truck was giving me issue, but no engine check light. Curious me, plugged in a code reader today and low and behold, P0420 is present. ( check engine light bulb prob burned out. )

No other codes are present. Decided to check the freeze frame data and it said:
STFT B1% -1.5
LTFT B1 % -2.3
STFT B2% 3.1
LTFT B2% -3.0

So for shits n giggles, I decided to do a live data on the way home. Everything appeared normal, I think, they way I understand it anyways, until I had to stop for a train.
While I was sitting there, then engine began to sputter and when I looked, I had o2 B1 v stuck at 0.200 and STFT B1 S1% stuck at -23.3

Those numbers stayed there for about 30 to 40 seconds as I tried to drive off, poor acceleration and sputtering. Then, the sputtering quit, and away I go, and those two numbers went back to what ever they had been up to that point and didn't do it again all the way home.

Would I be correct in assuming, based on the info, that the up stream o2 sensor has gone bad? Truck has 170000 kms and I assume the sensors have never been changed, and I see for recommends around 100000 km before changing.

98 mazda B4000 4x4 auto 4.0 V6

 

[RonD]

P0420 Cat converter not cleaning exhaust like it should, may be time for a new CAT Converter

1995 and up V6 Rangers have a 3rd O2 sensor after the Cat converter to check that it is working, this is the Downstream O2 sensor, B1 S2, Bank 1 sensor 2
The 2 upstream O2 sensors are what the computer uses to fine tune fuel trim(STFT-short term fuel trim), there is one on each exhaust down pipe
Bank 1 is passenger side, Bank 2 Drivers side, both are Sensor 1
B1 S1 = passenger side upstream O2 sensor

O2 sensors last about 125k miles, recommended change time is at 100k
O2 sensors use a chemical to detect Oxygen in the exhaust, this chemical gets used up and O2 sensors start to fade, much like a car battery uses chemicals to store a charge, chemicals get used up and battery starts to fade, 5 to 7 years, O2s last longer, lol, but they still start to fade

If you have 125+K miles on the O2s then yes, change them, MPG will improve so they will pay for themselves over the next 125k miles

What it looks like is that the MAF sensor needs to be cleaned or replaced, try unplugging MAF sensor and drive truck normally, then see how the STFT reacts when stopped.

O2 sensors are not used at idle(or when engine is cold) the computer should run engine Richer +5 to +7 STFT at idle.

-5 to +5 STFT is normal, older engines are often +3 to +10 because fuel pressure has dropped a bit, or some vacuum hoses have cracks.


Yes if CEL(check engine light) doesn't come on when you turn on the key then bulb is burned out.
P0420 is an emissions code so could turn on the CEL, but not all codes turn on the CEL right away, computer waits for the same problem to repeat a few times before it turns on the CEL.

 

[RonD]

Hi Ron. Thanks for your reply to my thread question. Sounds like you know a little something about the readings and how it all works.

I am slowly learning myself.

A little over a year ago I bought the truck. Long story short, had the heads rebuilt because of a burnt valve and a messed up valve job by someone else. The truck also had an over oiled K and N filter on it which I junked.
While the engine was apart, I checked, and cleaned everything including the maf sensor. It was full of oil.

Everything was fine, and about 2 months ago, truck lost power, then ran fine. Every now and again, it would stutter. Checked fuel pressure and replaced fuel filter. Pressure at 68 psi.

After plugging in the reader, and discovering the code, here's what is going on.

Most everything looks to be within spec, but the B1 S1 is all over the map. Seems to almost be following the B1 S1 sensor. S1 goes to .800 S2 goes to .800 S1 goes to 200, S2 drops to 200, etc.

So I am going to guess, like you said, the cat could be failing, or at least not working at 100% and or sensors need to be replaced.

What was alarming tho, is, when the truck stuttered the other day B1S1 went to 0.200 and froze there for a good 30 seconds and the STFT did the same at -23.3

They stayed there while I tried to get up to speed, and when they started moving again, truck ran fine again.

While I didn't record the numbers, the B2 s1 and B2 stft numbers continued to move up and down, and never went to that extreme.

So I guess my question would be, is, if the cat or the B1 S2 sensor is bad, would that affect the B1 sensor and stft? Or maybe the B1S1 sensor is toast too?

I don't want to throw a 1000 bucks worth of parts at this, and just be guessing. Going to do a couple more checks and tests yet this weekend, including a heat temp on the inlet and outlet of the cat.

Any thoughts
Thanks
juice90

Yes, downstream, sensor 2, after Cat, should show lower voltage, .1-.2 all the time if Cat is working, and be steady, P0420 can mean it is staring to change voltage more so Cat isn't working full time.

Upstream, sensor 1's, should be very active, but stay between .2 and .8 when cruising along, .1 is very Lean, .9 is very Rich.
O2 sensors can only "see" Oxygen, so Lean means lots of oxygen, Rich mean little oxygen, in the exhaust.
Cat converter should cause very Lean exhaust, mostly oxygen, pollutants are burned off, so stable Lean/low voltage O2 reading
One of the reasons computer changes Fuel Trims so much is so Cat can work better.

"To operate efficiently, three-way Cat converters need an air/fuel mixture that alternates between rich and lean. When the air/fuel mixture is rich, it reduces the amount of oxygen in the exhaust. This allows the reduction catalyst to break down NOX. But to burn HC and CO, the oxidation catalyst needs more oxygen so the air/fuel mixture has to go lean. This allows the catalyst to momentarily absorb oxygen and trigger a reaction that burns up the HC and CO."

So there is a "method to the madness", lol.

The computer could be trying to make the Cat work better, hard to say, but certainly seems like Cat is done and needs to be replaced.

The oil in the MAF and so in the engine and exhaust can shorten Cat's life, also O2 sensor life.

I would start with replacement Cat, and see how downstream and upstream O2s react, see if the fuel trims become more stable