how to check O2 sensor?

[erikandelman]

Hi all,

How do I check the O2 sensor with a good multi meter? What's it supposed to read on a 88 Festiva? I assume the car should be warmed up.

Is there a way to adjust the carb mixture based on the O2 sensor reading?

Thanks,
Erik

[batstiva]

Do you happen to have a Hayne's? There are instruction in there on how to test the O2 sensor, but they're kind of long and drawn out to type on here.

As to adjusting the carb for the O2 sensor, that's what the ECM is supposed to do. If you've got a faulty O2 sensor, and you try to adjust the carb around it, you're just going to cause more problems and piss off your ECM.

[erikandelman]

I've got a Haynes here... but I swear it's not in there. The closest thing is a small section under Fuel and Exhaust, "emission control components" but O2 sensor is not mentioned.

[batstiva]

Section 6-10 in mine has how to test, and how to replace. With pics of the connector you need to probe. Yours doesn't have this?

[erikandelman]

Hmmm... mine is missing sections! Will have to look for another one.

[batstiva]

Hmmm. Didn't know there were two versions. All three of mine have that section, with most of a full page write up on checking/changing the O2 sensor. You sure it's not in there?

[erikandelman]

yeh, I've got the darn bootleg PDF version... I see that sections 6-8 are missing. I searched the forums for instructions but dind't find. Will have to bite the bullet and buy the book.

[jglutz]

From experience the Hayne's test will always be preceeded by the Check Engine Light comming on. More of and after the fact test.

The sensor..in technical terms...is un-bridged...meamanin it produces a millivolt output non proportional to the O2 contnet...or in laymans terms...something an average mulit-meter will not detect.

On a small multi-port like the festiva...a O2 sensor on the way out will start to result in idle 'chuggle' when the engine is at temp. the RPM will be steady at ±750 and the lunge down to 600 or so the kick back up...this will repeat itself every minute or so.

What's happening is the ECU is not satisfied with the O@ content per the air/fuel mixture being delivered tot he engine and the ECU makes an adjustment to see if the O2 sensor responds...the reason for the drop oin RPM. The O2 sensor responds and the ECU dos not turn on the CEL but the problem is still their.

If the exhaust is extremely black...rich mixture...or the fan runs alot at idle....lean mixture...and the CEL is not on...the O2 sensor may be on the way out.

Anyway...with connecting to the ECU or extacting a flash code...it's almost imposible to diagnose a bad O2 sensor.

[ejp2fast]

related, but slightly off topic....

how do those devices that turn off your CEL light and "trick" the ecu in thinking the O2 sensor is oK work??

[FestYboy]

they don't turn off the light, you do, those units just send a swinging voltage back to the ECU and the ECU thinks things are OK.

Erik, the O2 in your case should sit around .450 VDC with a good DVOM. i'm not too sure it should swing being that the car is carbed and it has a passive AIR system. i believe it's just used as a monitor to make sure things don't get out of hand. i DO know this: DON'T use bosch sensors, they don't put out the correct signal/voltage for our ECUs, you need to use the Denso unit.