P0420 after long drive?

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KGMtech

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We went to Connecticut this weekend, round trip of about 1700 km (1060 miles) from our home in SW Ontario in the old 2009 Ford Flex, 3.5V6 not boosted. Check engine light came on in the last hour of driving ~ car drove flawlessly whole trip. P0420 is Catalyst System Efficiency Below Threshold Bank 1.

Background: Car has 195,000km, both O2 sensors done at 155,000km, all 6 sparkplugs done at 168,000km. Car runs fine, it's had Pennzoil Platinum 5W20 and FL500S filters 2x per year.
When I did the O2 sensors, I peeked down the open hole, the cats looked good, all honeycomb intact from what I could see.

I cleared the code, I'll wait to see if it comes on as my youngest son drives it for his school.

Any suggestions what it might be related to? bad gas? driving too fast into a strong wind?

Thanks, Ken.
 
Doesn't this have four o2 sensors? If you changed the upstream ones you should change the downstream as well. Use Motorcraft or Denso NOT Bosch o2 sensors. Stick a bottle of Guaranteed to Pass or Cataclean in the tank because why not. Check your fuel trims, make sure it's not running rich or something. Until you have to worry about smog inspection if you have that there, don't worry about it otherwise, a worn out cat won't hurt your car.
 
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Originally Posted by dogememe
Doesn't this have four o2 sensors? If you changed the upstream ones you should change the downstream as well. Use Motorcraft or Denso NOT Bosch o2 sensors. Stick a bottle of Guaranteed to Pass or Cataclean in the tank because why not. Check your fuel trims, make sure it's not running rich or something. Until you have to worry about smog inspection if you have that there, don't worry about it otherwise, a worn out cat won't hurt your car.


Yes it has 4 sensors. Only replaced the upstreams because they do most of the work. I used NTK sensors - I know the Bosch ones are not so good. Maybe I should wait to see if the code comes back before I replace bank 1 O2 sensor with Motorcraft or Denso.

Our emission testing program in Ontario was just scrapped in the last provincial election. Too few cars failed - the public felt it was just an annoying tax grab every 2 years.
 
Originally Posted by KGMtech
Originally Posted by dogememe
Doesn't this have four o2 sensors? If you changed the upstream ones you should change the downstream as well. Use Motorcraft or Denso NOT Bosch o2 sensors. Stick a bottle of Guaranteed to Pass or Cataclean in the tank because why not. Check your fuel trims, make sure it's not running rich or something. Until you have to worry about smog inspection if you have that there, don't worry about it otherwise, a worn out cat won't hurt your car.


Yes it has 4 sensors. Only replaced the upstreams because they do most of the work. I used NTK sensors - I know the Bosch ones are not so good. Maybe I should wait to see if the code comes back before I replace bank 1 O2 sensor with Motorcraft or Denso.

Our emission testing program in Ontario was just scrapped in the last provincial election. Too few cars failed - the public felt it was just an annoying tax grab every 2 years.

NTK is fine. I'd say check the fuel trims and change the downstreams and don't worry about it if the code comes back.
 
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Dont worry about changing the downstream. Just get a spark plug anti-fouler and drill it so the sensor can fit and screw it back it. Acts as a spacer and reduces the amount of exhaust the sensor is exposed to. Itll keep the light off.
 
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Originally Posted by Colt45ws
Cat is done. You have fairly high mileage. Theres not much you can do besides replace it.

Not necessarily... you will have to do some further diagnosis first.

I resolved my 0420 by replacing the downstream sensor.

My cats are fine.
 
Originally Posted by Colt45ws
I think thats the first time Ive heard of that working.

When you receive a P0420 code it means your catalytic converter is not operating at maximum efficiency. Usually this means your catalytic converter needs to be replaced or there is a problem with your oxygen sensor providing inaccurate feedback to the computer (which sets off the P0420 code).

You can investigate further to see if your cats are working by using a laser thermometer.

Warm up your car to operating temp and check the temperatures of the pipe BEFORE and AFTER the cat. A properly operating cat should be hotter AFTER the cat.
 
Originally Posted by Colt45ws
Cat is done. You have fairly high mileage. Theres not much you can do besides replace it.
200000km is not really that high mileage. My old car used to throw them every so often, unless throws it constantly it could just be a degrading sensor + cat combo for a split second.
 
Takes much longer than a split second. Theres a running average in the PCM of (and Im going off of memory here) the ratio of switches from the US vs DS sensors. So a 1.0 would mean the DS is switching indentically to the US which would indicate a very failed or non-existant cat.. So your average was ending up at the cutoff (.6 to .8?) and depending on day to day variations it might have pushed it over the trip point. Thats why it would be odd for a new DS sensor to fix it. A sensor is typically not going to fail by generating 'extra' voltage and switching more often.
 
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Originally Posted by Colt45ws
Takes much longer than a split second. Theres a running average in the PCM of (and Im going off of memory here) the ratio of switches from the US vs DS sensors. So a 1.0 would mean the DS is switching indentically to the US. So your average was ending up at the cutoff and depending on day to day variations it might have pushed it over the trip point.
Very insightful! I've never read anyone explain that.
 
Don't just assume it is the 02 sensor though -- you also need to check for exhaust leaks aft of the sensor at bank 1 sensor 2. P0420 is a generic code that hits a lot of vehicles.

You also need to look at live data that is bank 1 sensor 2 and graph it compared to bank 1 sensor 1. Don't just throw parts at it. You could replace bank 1 sensor 2 and still get p0420 because of an exhaust leak either at the bung or in the pipe near the sensor. I had my mechanic weld on a new bung and a bit of pipe coming out of the cat and new clamp, and reinstalled the same sensor the problem went away. More than a year later it never came back. It was definitely not the catalytic converter or even the sensor.
 
Originally Posted by maxdustington
Originally Posted by Colt45ws
Takes much longer than a split second. Theres a running average in the PCM of (and Im going off of memory here) the ratio of switches from the US vs DS sensors. So a 1.0 would mean the DS is switching indentically to the US. So your average was ending up at the cutoff and depending on day to day variations it might have pushed it over the trip point.
Very insightful! I've never read anyone explain that.

Here it is straight from the Ford OBD manual. I finally found it.

Quote

Under normal, closed-loop fuel conditions, high efficiency catalysts have significant oxygen storage. This makes the switching
frequency of the rear HO2S very slow and reduces the amplitude of those switches as compared to the switching
frequency and amplitude of the front HO2S. As catalyst efficiency deteriorates due to thermal and/or chemical
deterioration, its ability to store oxygen declines. The post-catalyst HO2S signal begins to switch more rapidly with
increasing amplitude, approaching the switching frequency and amplitude of the pre-catalyst HO2S. The
predominant failure mode for high mileage catalysts is chemical deterioration (phosphorus deposition on the front
brick of the catalyst), not thermal deterioration.

In order to assess catalyst oxygen storage, the catalyst monitor counts front HO2S switches during part-throttle,
closed-loop fuel conditions after the engine is warmed-up and inferred catalyst temperature is within limits. Front
switches are accumulated in up to three different air mass regions or cells. While catalyst monitoring entry
conditions are being met, the front and rear HO2S signal lengths are continually being calculated. When the
required number of front switches has accumulated in each cell (air mass region), the total signal length of the rear
HO2S is divided by the total signal length of front HO2S to compute a catalyst index ratio. An index ratio near 0.0
indicates high oxygen storage capacity, hence high HC efficiency. An index ratio near 1.0 indicates low oxygen
storage capacity, hence low HC efficiency. If the actual index ratio exceeds the threshold index ratio, the catalyst is
considered failed.

I was closeish with my explanation.
 
It does not matter what scientific explanation is. There are some people who sincerely believe that changing O2 sensor will fix their P0420/P0430 problems. Let them learn the hard way.
 
Unless you can watch the waveform of both fwd and rear sensors, your not going to get far. And yes anti foulers work when you build them right lol. Verify proper voltage range of the rear o2 if within normal range, sensor is good. or always swap bank one to bank 2 and see if code follows or stays put. simple trouble shooting save you a ton of wasted money
 
Originally Posted by dogememe
Use Motorcraft or Denso NOT Bosch o2 sensors.


Why not? Any data whatsoever showing them to be less functional than any other? Don't say the proof is on internet forums where lots of guys say so. DATA!
OP. I have no idea what Canadian laws are but call the dealer, they may warranty the cat.
 
Originally Posted by Trav
Originally Posted by dogememe
Use Motorcraft or Denso NOT Bosch o2 sensors.

OP. I have no idea what Canadian laws are but call the dealer, they may warranty the cat.


In Canada the emission system is covered to 8 years or 130,000 km whichever comes first. This Flex is way over for km and 1 year over for time.
 
Check the 02's on a scan-tool before firing the parts cannon at it with a Cat or new 02's. Use OE part brands for whatever you replace if you are going to keep it as there is way too many headaches using off-brand from the original equipment has been a lot of our experiences when it comes to emission components.
 
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