ford focus post cat o2 static voltage p0420

2004 ford focus 1600 cc 16v european model

the trouble light came on , p0420 , ive looked at live data and the post cat o2 voltage is fliciking around 0.1v , i let the parts cannon go and new sensor does the same , no leaks in the exhaust

we have to pass a local emissions test here in the uk , i had my test center run the test and emissions wise the cat is working and fine with good clean emissions

thats about the end of my expertise,....what now ? toot some carb cleaner into the inlet and see if the voltage rises?

appreciate any tips on what to look at next thank you

Unplug that post O2 sensor and put B+ to the signal wire with a testlamp, what does the scan tool report for a voltage?

The Lamp should not light, if it does you've a short to ground in the signal wire. Further checks will be required.

great will report back when the weather clears up - thank you

**edit should i disconnect the ECU or no?

Don't unplug the ECU, the scan tool should read the maximum volts it was programmed to read, which might be as high as 1.2 volts, It will disregard the other 10.5 volts.

The point of this is to determine why sensor 2 is fixed low.

okay did your test , unplugged post cat 02 , car side of harness signal wire does not light a test light

please advise on next steps , thank you

okay tested again today , seems its stuck at 0v now

i gave it a toot of carb cleaner and no change ..can see on the upper red line the pre cat sensor responded , green is 0v

guessing the signal wire maybe broken, what do you say?

okay did your test , unplugged post cat 02 , car side of harness signal wire does not light a test light

BUT, when you applied B+ to the signal did the scan tool report RICH or 1 volt?

If not, you need to unplug the PCM and test for B+ from the sensor harness end, or put a bulb in series with the signal to check the wires integrity.

BUT, when you applied B+ to the signal did the scan tool report RICH or 1 volt?

i didnt check for this^

will a regular test light suffice or rig something up with a 10watt bulb or such like?

I thought you would have tried to drive the signal HIGH to see wha the scan tool reports. If you can drive it High this proves the PCM can read the signal and the signal wire is good.

1. Unplug the sensor and apply B+ to the signal wire, what does the scan tool report?

If nothing, do the test I have in the picture to verify the integrity of the signal wire.

yeah im not confident enough to try stuff like that myself ,,in my head that would maybe damage the ecu

will try what youve told me , thank you again

Applying 12 volts to an O2 sensor signal wire(input) certainly will not damage a PCM.

It's the Heater circuit(output) you need to be carefull with, when performing by-pass testing.

The whole point of this testing is to:

1. Verify the PCM can "see" a higher voltage; because you are fixed low.
2. The signal is not shorted to ground.
3. The Signal wire is intact from the sensor to the PCM.

Good Luck!!!

yeah great , its not something i would have tried

will report back soon

Just an exerpt from Mr. Danner's book!!!

okay im back

to recap - P0420 little to no signal from post cat 02 , tried 2x new sensors much the same

ground in harness checks out with test light to 12v+ (should i load test with a bigger bulb?)

heater circuit in sensor checks out

12v+ into the car side of harness signal wire sends the scanner reading high as per your last comments ...see 2 stabs with test light on photo

other pic shows little /no output from sensor 2 , im guessing i need to bench test the new sensors?

thanks again

Alright, if you applied B+ to the signal wire with a test lamp and it did not light and the scan tool reported maximum voltage.

The signal wire is good, it's not open or shorted to ground.

Is the pre-cat sensor capable of displaying Rich/Lean voltages? The fuel trim is good?

If the signal wire is good, you tried 2 different sensors, the only circuit left is the heater circuit.

If the heater circuit is compromised it would take a much longer time for that sensor to read Rich and to start switching.

Inspect both heaters wires, I'm gonna guess they are ground side switched by the PCM, if so you'll have fuses for them in the underhood fuse box more than likely. One wire should be full time B+ and the other the ground. Use a test lamp to identify the wires.

You can always hook your test lamp in series with the pre-cat sensor heater circuit to verify "good" operation, then repeat the test on the post-cat sensor to see if the results differ.

The last possibility is both those new sensors were bad. But the wiring checks must be done before confirminf that.

thank you , i will check the heating wires asap

okay small parking light bulb into an old connector - no light on cold start ,or will the ecu know i have disconnected it , try a backprobe instead?

ECU will know if the key was one when disconnected.

Make sure there is no heater set, that would disable the driver.

Even if you disconnect and use a bulb in series the ECM may not like what it "see's".

So try both methods, unplugged and backprobed.

ECU may know if the key was one when disconnected.

Make sure there is no heater circuit code set, that would disable the driver.

Even if you disconnect and use a bulb in series the ECM may not like what it "see's".

So try both methods, unplugged and backprobed.