P0420 diagnosis

All right. 1st note is the screen shots are too low resolution to be legible.

I would look at fuel trims while idling (hot) and at cruise. Short Term Fuel Trim is a direct response to upstream AirFuel sensor readings. (+) FT values means an increase in fuel while (-) values mean a decrease. Long Term FT tries to keep STFT near 0. ST plus LT equals total fuel trim.

Total trim values of 0 +/- 5% are great (my 06 at 185k runs 0 to +3%). 0 +/- 10% is usually acceptable. (I've seen cars at +17% that were "good enough" with no driveability problems, and I've seen "broken" cars at +9%). At 20% you start seeing codes.

Voltage at the A/F sensor doesn't tell you much. The ECM tries to maintain 300mV difference across the sensor. The exhaust mixture determines how much current the ECM has to supply in order to keep the 300mV steady. A 14.7:1 ratio is 0mA. Normal driving is about +/- 0.2mA (a couple hundred microamps). Fuel cut goes to +2mA(?) (can never keep the polarity straight).

Anyway, the engineers decided to display a voltage value instead of current. So the scan data shows about 3.3V at stoichiometry (14.7:1). Richer mixture shows as a lower voltage, while leaner mixture is higher (up to 5.0V at fuel cut- engine off).

The downstream S2 sensor is a normal "narrow band" type. It outputs near 0V when very lean/engine off and near 1.0V at its rich maximum. At typical steady state cruise or idle with the ICE and cat at operating temperature, you should see about 700-ish mV (650-800) on the S2 when things are ok.

You can get some idea of how good or bad things are by looking at the OBD monitor test results. If I recall there is a magnifying glass icon or somesuch for each of the monitors. Post the results for the catalyst and oxygen sensor tests and I can look at my 06.


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So if you look at each individual line/test, there is a Min, a Max, and a Value. When the ECM runs a particular test, it stores the result as the Value for that test. If the Value is between the Min and Max (within programmed limits), then that test passes.

"But wait a minute" you say, "all of these results show PASS but I still get a P0420." Ahh. This is where it really helps to have "known good data" - and some experience with OBD2. On my 06 all of the test results have similar values, EXCEPT for the catalyst test. I have a value of 0.119 with a MINIMUM of 0.110. Your passing cat test had a Min of 0.024.

After a code clearing the ECM can run some tests in stages- one quick test and then a later, more involved test. Your Value of 0.061 passes the first test but would fail the second test.

Your fuel trims (if they stay consistent with your posted numbers) indicate good A/F and O2 sensor function. Your monitor test results also indicate good sensor function. Which leaves a bad cat.

Might want to do a "thorough" leak test to rule out that possibilty. I like to safely raise and support the front of the car. Start it up in maintenance mode or turn the heater way up. Have a second person use a heavy leather glove or thick towel to cap the tailpipe AWAP while I listen from under the car for leaks. (An "open ended" stethoscope or piece of small hose/ tubing with one end held to an ear works). ANY hiss or noise can be a leak- you want to hear NOTHING from the cylinder head on the engine back to 18 inches past where the downstream O2 is.

The ECM does something called the Oxygen Storage Capacity test to figure out if the catalyst is up to snuff and can do its thing. Besides platinum, palladium, and/or rhodium (to allow reactions to happen), the cat has cerium in it to store oxygen (when lean) and release oxygen (when rich) so that the HC and CO has something to react with.

ECM shifts fuel control rich- which uses up any O2 in the cat. Then it shifts lean, and watches how long the downstream S2 O2 takes to indicate lean after the upstream S1 A/F does. A healthy cat can store lots of O2, so the S2 can take "quite awhile" to show lean as the cerium "grabs onto" all the oxygen it can hold. A deteriorated cat will let the S2 go lean sooner.

You can use Techstream to perform a similar test. Drive the car (hard-fast) to get the ICE and cat up to operating temperature. Have Techstream ready and hooked up. Park on a level (safe) area with parking brake set. Foot on the brake and gas. With the engine running _ shift to neutral then release the pedals. The ICE should stay running at about 1000 RPM. (The inverter can't charge the HV battery like this so don't let it stay like this for lots of time- I like to start with battery at 6-7 bars).

In the ECM heading of Techstream there is a test to change fuel trim. You need to watch the A/F S1 and O2 S2 voltages while doing the test. Ideally you would graph just those two data PIDs (IDK if you can do that on Tech stream while performing the test). Then start the test. When I click on "+25%", A/F on mine is around 2.0V, and O2 is 0.96V. After around 7 seconds I click on "-12%" . A/F quickly goes to 3.9V- count the seconds until O2 shifts down, 0.04V for me after 5 seconds (helps to take video).

This is not the actual monitor test, but is a good comparitive test. A new or very good cat can take 10 seconds or longer. When S2 O2 shifts at 2-3 seconds or less I usually see P0420 codes set. If I want to repeat the test I exit the test, shift into park, and run the ICE at high rpm for a minute then begin again.

This test lets you check S1 A/F, , S2 O2, and cat function.


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I mostly use a $nap-On scantool for day to day work. It lets me reduce the number of data PIDs that are being displayed- which speeds up the refresh rate. I don't know if you can do that with Techstream while performing a bidirectional test.

In general I avoid AM cats if at possible - I've had too many comebacks and the fit can be pretty bad on some. Maybe others here have some recommendations.

One thing, if your engine uses "a lot" of oil, then any cat won't live for long. More oil it drinks and burns, the sooner the cat will be rendered inert.

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