Hi All,

I have a Seat Toledo MKII 1.6 16V (2002) and a code: P0420.

All I see in I/M readiness is that everything is green,
only the oxygen sensor is incomplete and the only problem
in test results is the following:
Test report:
——————
TID: CID:
– Rich to Lean sensor threshold voltage(constant)
Max: 5,000
Test result value: 8,333
FAIL

I have 2 sensors, one before the cat. converter (5 wires,
wide band) and one after the cat. converter (4 wires).

I dig Google/Bing/Yahoo already, lots of forums, I asked
lots of mechanics as well.. Some of them says it is really
the catalytic converter, some of them says it is the O2
sensor.. but which one? I think the one before the cat.
is a AFR sensor but the other is a simple O2 sensor. Which
one is reported “faulty” in Torque?

The weirdest thing to me is: all I/M monitors are green:
Continuous Monitors
– Misfire [OK]
– Fuel System [OK]
– Comprehensive Component [OK]
Non-Continuous Monitors
– Catalyst [OK]
– Heated Catalyst [OK]
– Evaporative System [OK]
– Oxygen Sensor Heater [OK]
– EGR [OK]
..except the Oxygen (O2) Sensor!
How is it possible that the catalyst readiness monitor
is green, passed – but the O2 sensor is “incomplete”?

_______
Norbert

Yes, I know what P0420 means but I am really interested in
the “other story” since “Rich to Lean sensor threshold
voltage(constant)” is the only thing that fails and in
readiness all tests are OK (even the catalytic converter
test..) but the oxygen sensor..

The HO2S B1S2 in idle is around 0.1V, when driving it is
fluctuating between 0.1V and 0.9V. There are no driveability
issues anyway, no rough idle, gas mileage is not that bad.

I just don’t understand how come that the readiness test of
the catalytic converter is OK but the oxygen sensor test is
not – as far as I know the cat. test should not run until the
ECU obtains evidence that the oxygen sensor is reliable.

_______
Norbert

@crok

The ECU runs all kinds of test on operations of the various systems it monitors and controls – some continuously and some periodically. The results of many of them are kept by the ECU and reported when Torque requests through Mode or Torque Scan. I do not know (and doubt anyone does) just which tests are ran instantly when you tap “Test Results” Scan!. But certainly not all of them — (ie: Evap Monitor.)

Your TID and CID are blank (probably dollar/sgn 02 & 01) but this equates to a specific “TEST”, not a specific sensor. (in this case the system being tested for efficiency includes the pre-cat O2-1, the Cat, and the post-cat O2-2).

The ECU can determine “catalyst efficiency” by adjusting richness ( + – fuel trim) of the mixture (as sensed by O2-1) and see if the post-cat O2-2 reading is within spec. Likewise, by switching from rich to artificially lean – the ECU can determine “how long” it takes for O2-2 to realize (register) the change (drop to the proper voltage constant), or whether it EVER REGISTERS it – and sets a P0420 code. (ie: if the Cat is bad or O2-2 takes too ling to register “rich to lean” transition, the test fails).

In your case the TEST result was 8,333. It says the Max should be 5,000. I do not know what the units are (millisec’s / microsec’s or millivolts or what), but it says 8333 > 5000 = Fail. In my humble opinion, its probably a “lazy” O2-2 sensor. Your vehicle is a 2002. No indication of mileage or if O2’s have ever been replaced.

I know their ability to react does deteriorate with age and other factors. And the ENTIRE OBDII system is based on them producing .45 volts at a perfect fuel/air ratio of 14.7 to 1. Will their calibration drift with age?? Personally, I believe it does drift toward lean – causing the ECU to adjust mixture richer (hurting your mileage). And I believe in replacing O2 sensors as a preventive maintenance. See the article at:
http://www.obdii.com/articles/Replacing_Oxygen_Sensors_Preventive_Maintenance.html

You posted some voltage readings, but graphs or waveforms are much more meaningful. You can set up a Torque screen to display graphs gauges for pre-cat and post-cat O2 sensors together. Monitor their signals under different conditions to help determine what they are doing.

On cold start, after the ECU goes into “closed loop”, – you should see O2-1 cycle above-below .45v. Before the CAT “Fires” (gets hot enough to sustain constant chemical reaction efficiently) the O2-2 signal will track the pre-Cat O2. (This condition also helps the ECU determine if “soak time” was sufficient to determine a new “drive cycle”.)
These graphs are from NEW O2’s on my ’04 F150 5.4L Triton, original CATS w/ 201K miles.

Cold start after closed loop……

These graphs are while gentle cruising……

These graphs are at time of deceleration…..

The bottom line: If your CAT doesn’t rattle when you “bump” it (cold of course), I would definitely go for new O2 first if it is a 10 year old 100,000+ mile car.

Hope this helps.

@crok:
I would feel terrible if I mislead you to unnecessarily replace parts. Lord knows I have done my share of that. LOL So, my feelings will not be hurt if you double check my information and get other opinion.

But, if B1S2 is doing basically the same thing as B1S1 – I would say that tells me that B1S2 _is_ working, and the CAT is _NOT_. There is no reliable “steady” number (0.65v), or even relatively steady number I can tell you to “expect” seeing from B1S2 due to the many variables at play. But if the cat is doing ANYTHING AT ALL, the “aft” oxygen reading must at least be “DIFFERENT”, and should generally bobble very lightly somewhere around .6. – .7v. On occasion, I see mine do crazy things that I can not explain. (perhaps ECU “TEST” routines, or engine misfires, or temporarily fluctuations in Variable Cam Timing or something). If you have a laser thermostat, you might check the temp of the CAT housing – should be between 500-800 degrees if it is functioning. Being well below that would tend to confirm the conclusions drawn from the B1S2 reading.

I know an extended over-rich operation can “overwhelm” a CAT, coating the catalyst with carbon and pollutants and KILL a CAT. (THIS condition should cause the MIL to flash – warning of existence of a CAT damaging condition). BUT, Before changing the CAT, I would set up a Troque screen and monitor Short Term and Long Term Fuel trims, engine coolant Temp etc. Verify no vacuum leaks. Make sure the coolant temperature gets up to proper EOT normally. Address and remove any problems that would cause over-rich condition.

When the condition is corrected or returns to normal (or 02’s are replaced), if the CAT chemical reaction can “light off” and start working AT ALL – It can sometimes cure itself with time at operating temperature (sorta’ like a self cleaning oven or incinerator). If this is a possibility – you might take the vehicle on a road trip for 100 miles or so at road speeds, then repeat your tests.

If no change – my opinion is the CAT is dead.

Good luck. Let us hear what happens.

Let us know. Our fingers are crossed as well.

hum i have no experience in this personnaly but like you said
after reading the text a faulty liquid temps sensor or thermosat or near to blow engine lol

could set that code

First of all thank you for the kind words, I really appreciate
it! I’m a network technical design architect but my first pro-
fession was mechanical engineering – so I am more-or-less fami-
liar with these things, but really not a car mechanic.. just
want to see everything working as designed (:

I had the cat. replaced by a car mechanic – so this is why the
after-cat-sensor reading is now looks good (during cruising
I see now steady 0.6..0.7V) and I saw no P0420 since the rep-
lacement.

Yesterday I looked for the coolant temperature sensor, loosen
up and readjusted almost all possible cables and connectors.
Hope it was enough.. but I’ve noticed, that when the engine
is in idle state the engine’s idle rev. is fluctuating a little
bit – may be 30..50rpm, between 680..730. Not always, but some-
times. Steady for minutes and starting “waving” for 30..40secs.

In Torque I found that “Fuel gauge” that displays when the O2
sensor (the before-cat-sensor) is used to inject fuel. So, I
started the engine and watched how much time dows it take for
the O2 sensor to produce usable voltage: maybe after 20..25
seconds the fuel gauge said it is closed, so I started to go.

So far so good – after a day I did not see the MIL/CEL light.

I guess, that your car has only 4 wire O2 sensors (you men-
tioned that it would be a good test to swap the sensors) but
mine has a 5 and a 4 wire sensor (the first one is a A/F and
the second one is a simple exhaust gas diag. sensor) thus I
cannot swap them with each other :\ and unfortunately I don’t
see sensor heater circuit current test in Mode 6.

One of my colleagues gave me a VCDS cable so I will check the
PIDs in my car’s computer – may be I will see something usable.

Next step: monitoring.. keep a close eye on the O2 sensors’s
voltages, the I/M readyness values and tests, the O2 tests..
fuel trim graphs and closed/opened state..

I’ll keep you updated!

(PS: I’m not a native english speaker so sorry if I misspell
or use a term improperly.. et cetera..)

@crok;

I responded via a new thread, because TOPIC of this thread has drifted from the title. Please see “Occasional – but avoidable P0135 (O2 heater ckt err)” posted this date.