o2 sensor expert help needed

[9000rpm]

I'm trying to get to the bottom of the high co on my car and I'm using some old autodata books to help me check for things. One of which is testing the o2 sensor by bridging pins on the diagnostic port and measuring the voltage across other pins. If I do this and measure the voltage I'm getting around 3v when the sensor is warmed up and 5v when the sensor is not warm. The books says the voltage should be 0v when warm, if its not the sensor needs replacing. I though o2 sensors produced between 0v and 1v when working so confused how its producing 3v even if it is faulty.

Anyone know what going on and what I should see when testing using the diagnostic port?


Cheers

Paul

[jon]

Are you measuring the voltage across VF1 and E1 or OX and E1?

OX is the raw signal from the Oxygen/Lambda sensor, so it will fluctuate between 0v and 1v.

When TE1 is jumpered to E1, VF1 is a signal generated by the ECU after processing the OX signal. 5v richer than stoichiometric, 0v leaner than stoichiometric.

If you use a analogue voltmeter you should see the needle fluctuate 8 or more times in a 10 second period if the oxygen sensor is working correctly.

The proper Toyota test procedure (for a 3S-GTE, 3S-GE should be the same) can be found at http://www.lovehorsepower.com/bgb/mecha ... tem/34.htm

[9000rpm]

Hi Jon,

Thanks so much for taking the time to reply with such valable info.

I was following the autodata book where it first says to bridge TE1 and E1 then measure the voltage between E1 and VF1. unfortunatly I only have a digital multimeter so was hard to see if it was fluctuating 8 to 10 times a second. so I moved on to the next step of measuring the voltage with the bridge between TE1 and E1removed and the autodate book says it should read 0v but I was surprised to see a voltage of around 3V. the book says to replace the sensor if reading over 0v?

can I plug in my multimeter to Ox and E1 to see what the o2 sensor is doing? if so does TE1 and E1 need to be bridged?

[jon]

The autodata book testing procedure sounds the same as the official Toyota procedure, except the voltage between VF1 and E1 (with TE1 connected to E1) should fluctuate 8 or more times over 10 seconds, not 1 second! So even with a digital multimeter you should see it fluctuating, but not enough to count the number of fluctuations reliably.

The voltage between VF1 and E1 without TE1 connected to E1 indicates the the air/fuel feedback correction value. 0v means less fuel required than normal. 2.5v means no correction. 5v means more fuel required than normal.

You can plug a multimeter into OX and E1 to see the raw voltage output of the oxygen sensor, but again it will be switching between 0v and 1v, so you won't see anything particularly useful on a digital multimeter.

I'd recommend getting a £5 analogue multimeter from E-bay and running the 1st test again.

[mrfil13]

Hi Jon,

Ive gone through alot of this recently too and carried out the tests you have specified, although you maybe able to confirm, with the VF1 and E1, is that correction applied by the ECU in open loop as well as closed?

[jon]

The voltage measured on VF1-E1 is a course representation of the long term fuel trim which is applied in both open and closed loop.

[mrfil13]

Thanks for confirming that, its how i read it before but could never find much info on it.

[9000rpm]

cheers guys this is brilliant info.

I have just read the autodata book again and it does say 8 times in 10 seconds I read it as 8 to 10 times a second!

thats very interesing understanding what the voltage on VF1 and E1 means. so my 3v (think it was 2.88 ) means it running slightly rich for some reason but nothing too bad.

I now cant understand why the autodata book and that link says to replace the sensor if over 0v? I guess thats only if its fails the 8 changes in 10 seconds test and if it is changing more than 8 times in 10 seconds a voltage over 0v is quite normal. any idea why it is like this?

[mrfil13]

The way i read is that is that the test of over 0v but not fluctuating means its sending a voltage but its slowed/dead, if it was 0v then you would go down the route of checking connections.

What i could not find out was if its possible for an old Lambda sensor to be still responding 8/10 times every 10 seconds but for it to be still giving a false reading, i imagine it is possible.

Mine was reading 1.25v before, changed the plugs and cleaned the AFM and it was reading 2.5v last time i checked.

[jon]

I now cant understand why the autodata book and that link says to replace the sensor if over 0v? I guess thats only if its fails the 8 changes in 10 seconds test and if it is changing more than 8 times in 10 seconds a voltage over 0v is quite normal. any idea why it is like this?

I must admit it's not very clear why measuring over 0v is an automatic replacement of the sensor.

As I understand it:

If the reading is above 0v then the ECU is able to adjusting the fuelling correctly but the fact that the sensor failed the 8 in 10 seconds test means the sensor is reacting slowly and should be replaced anyway.

If the reading is 0v then the ECU hasn't managed to adjust the fuelling, there may be another underlying reason why it can't. The Toyota test procedure has various other tests to attempt to isolate the problem, not all of them result in replacing the sensor.

[9000rpm]

Cheers guys for all your help. I now have a better understanding of what is going on so will hopefully be able to diagnose the high co better.

at least I know the 02 sensor is working OK!

I have just found the intake air temp sensor was not installed in the air box and has been floating around in the engine bay reading the hot air temps of around the engine and not inside the cooler air box! I've now connected it back up. It could explain why its been jumpy in traffic! I doubt it explains the high co though

is it worth re-setting the ecu now the sensor is fitted correctly?

[mrfil13]

It might speed up the feedback correction but i didn't reset mine between testing, changing plugs, cleaning AFM and the correction getting back to 2.5. Maybe just take it for 15 minute drive and see if it gets back to 2.5v.