PO 420 almost always means the converters are toast, however you need to repair any other problems that may have caused them to be dammaged.

 

Yes, at this point I understand that. I'm not opposed to getting a new cat but from what everyone tells me, if I don't fix the original problem then the new one will go bad too. I actually took it to the dealership and all they told me was it needed a new cat.

I have a good scanner that reads the live data, but I'm not experienced enough to read the data and figure it out. If anyone could point me in the right direction to fix the problem that is ruining my cat that would be great. I can post more freeze frame data if that would help. Thanks

 

The fuel trim data is very useful. The values change according to load and conditions, so knowing the range is useful, and knowing both the LTFT% and STFT% is important. While there is other useful data, the fuel trim data gives the clearest picture of what might be going on.

Some maintenance items include:
•O2 sensors @ 80,000 miles
•Spark plugs need to be inspected and properly gapped @ 100,000 miles. Usually these are replaced instead of adjusted.
•Use a good engine cleaner drawn through the intake at least annually. I personally like Seafoam, but BG44K is a good product as well. I do not personally recommend adding it to the engine oil.
•Make sure the fuel filter is replaced regularly, low fuel pressure not only causes drivability issues, but can cause fuel pump failures, and can contribute to problems that can damage a catalytic converter.
•Check for intake leaks and make sure the PCV valve is functioning correctly.

 

Thank you everyone for your input so far. I have a feeling that maybe I'm not explaining the situation well enough so far. To date I have given it a full tune up, with spark plugs, air and fuel filter, and pcv valve. I have also replaced all 4 O2 sensors.

I beleive that it is narrowed down to either a fuel delivery problem, or some sort of vacuum leak. That's were I get stuck, I don't know how to narrow it down further. I have visually inspected it for intake and axhaust leaks but found nothing. But there could be a leak that I can't see. I could guess, but I really don't want to buy the wrong part and not fix the problem. And like I said, I took it to the dealership earlier and got no where.

I was hoping the freeze frame data would give me a good indication of what's going on, but I'm just not experianced enough to read it. Is there anything that would lean toward it being a certain problem. I'm very curious why it shows 4 short term and long term fuel trims on the freeze frame and 2 of them are negative 100.

However further down in the data it shows short term fuel trim with the O2 readings and 2 of the 4 short terms are positive 99. Does anyone know what this means.

Thanks again for everyone's help.

 

Check out this mechanic's video pertaining to P0420 (If the link doesn't work, just do an internet search for "scottykilmer fixing a bad catalytic converter P0420". (Pause for you to watch it).
Since your code has been intermittent, I think your cats are a good candidate for a cleaning as indicated in the video (or use Seafoam in the tank if it makes you feel better). Your cat's current efficiency may be right on the borderline such that the quality of the gas you last filled up with will trigger the code. Plus, if there is another root cause which fouled up your current converter, it will just foul it up again and not a brand new one.

 

Since you have already done all the tune up, replace the cat and be done with it.

 

Let me just be blunt. I am a ford tech day in day out. Everyone that comes through that has a p0420 or p0430 needs cats bank 1 or 2 whichever it sets plain and simple. the IM240 test on these new vehicles is very accurate if it sets these codes they are done. If vehicle is low mileage you must fix whatever caused them to go bad in the first place. Sorry to say its about to get real expensive. Ford cats are easily over a grand every time. I have heard of muffler shops welding in universal cats in place for a 1/5 of the cost. FYI

 

I'm going take a stab and resurrect this old thread as mine is also a'99 5.0 with a 420 code like the original post.

The engine has 185,000 miles on it, is bone stock and has recently had new plugs and wires installed.

The Check Engine Light (CEL) came on.

I have an Auto Enginuity (AE) and by no means do I know or use its capabilities. But I can set it up to watch all 4 O2 sensors.

Originally the B2S1 O2 sensor wasn't reading right. I don't recall if the voltage was high or low, but it was steady at one extreme. Not moving up and down like the other three.

I moved B1S2 into its place as it was reading good and the B2S1 position was harder to get to. I had the truck on ramps and chose to make do the hard work now, and when I got the replacement O2 sensor I could reach and change the B1S2 without ramps.

After this I ran looked at all 4 sensors on the AE and sure enough, B2S1 was moving like the others and B1S2 was steady.

New sensor arrived and swapped it into the B1S2 position.

Cleared the codes with the AE and ran the KOER test, then looked at the 4 O2 sensors...all were moving lively and the LTFT Bank One and Two and STFT Bank Two were all green. I did not note any of the O2 Sensor Fuel Trim at that time.

A week later the CEL came on again.

AE shows the LTFT Bank One and Two and STFT Bank Two are all green.

Both B1S2 and B2S2 Output Voltage are zero.
Both B1S2 and B2S2 Sensor Fuel Trim are pegged at %99.06. The S1 position for both are green.

For giggles, the the KOER test showed the:
Bank 2 Switch Ration and Max. Limit colored red at .827
Initial Tank Vacuum Bleed-up and Threshold colored red at -7.467
Vapor Generation Max. Pressure Rise colored red at -63896

Any chances this is something other than just plain worn out catalytic converters?

 

The B1S2 and B2S2 sensors are not supposed to pulse, their function is to monitor the CAT eficiency, normal reading should be about .4-.5 steady..

... Philip

 

Thanks for the input!

When the CEL lit the first time, the AE read that both S2 sensors were moving up/down as was B1S1. B2S1 was steady high or low, I don't recall, but made the determination that it was bad.

I swapped B2S1 with B1S2 and the steady readings and the up/down readings swapped locations as expected.

When the B2S1 sensor was replaced with new, it was reading up/down as were the other three. Cleared the CEL code and gave the wife the keys.

Philip, I think that all 4 sensors reading up/down goes against what you are saying?

About 2 weeks later the CEL lit again and now both S2 sensors ready zero. 0.0 specifically.

B2S2 is new, and B1S2 was operating fine before. Now both read zero. The CEL light is on and I have a P0420 code.

I will try the easier of the two cat cleaning methods suggested in the linked video and will mentally prepare to pull the cats off just in case.

Thanks again for the input.

 

There can be no argument, the spec is such that

the O2 sensors close to the engine MUST switch between .05 and .9, quickly with no bias (the ECU usues this info to maintain FR of 14.7/1),

the O2's after the CAT CONV must generate a steady reading between .4/.6, this reading monitors the condition of a working CAT, the reading will differ when the CAT is compromized...

... Philip