MRecio

MY truck threw a code for a lean condition from the O2 sensor I forgot the number. The weird thing is my truck is always running WAY to rich so im thinking its time to replace them. they have 150K on the stock ones for a 94 4runner. Well i checked out sparkplugs.com and well they have denso and NGK for about the same price the NGK being slightly cheaper at 85 bucks will one work better then the other? i know these are both great brands so im not sure which to get

my other question is do i need to replace the rear sensor as well? does this sensor affect the air fuel mixture? or is it just there as a check for the cat to make sure its working? I would love to only have to replace the front sensor but if the benifits are there i will do the rear as well.

Targetnut

The rear O2 sensor does not effect the mixture, it is there only to monitor catalyst efficiency.

check the output voltage of the front O2 sensor at the diagnostic connector "OXY 1" It should fluctuate back and forth between approximately 0.2V and 0.8V it should go back and forth at least 8 times in 10 seconds at 2500rpm.

The code you have may be due to degraded O2 sensor perfomance and if so should be evident with the above test, but it could be due to a problem with fuel delivery.

MRecio

exactly how do i test it? put the posative to the terminal and the negative to the battery ground? or exactly where do i put the + - of the volt meter? i cant get a reading like anything i should.

4Crawler

iTrader: (3)

Full test procedure can be found in a Factory Service Manual. But in short, usually a test pin in the diagnostics box (labeled OX or something like that). + probe there, - probe to ground and run engine @ 2500 RPM or so for a few minutes to warm it up. Then watch for at least 8 voltage swings between lean and rich within a 10 second period. Voltage swings are small, running from ~0.2 volts to ~0.8 volts or thereabouts. Usually need to use a good old analog meter (i.e. one with a needle) or a very good digital gauge or scope.

MRecio

thats what i did but at idle the volts were none to .001 and when i gave it gas it would shoot up to .02 at the most then back to .001 nothing at all in the .2 to .8 range... but it cant be right cuz something that off would surley throw a code... the code mine threw only stayed on for about 20 min then went off without turning the truck off and never came back.

Targetnut

If those are your readings, then there is something wrong.

Do you have an exhaust leak upstream of the O2 sensor? if so then the air that gets in the exhaust stream can cause low output from the sensor.

If no exhaust leaks are there then is the engine running smoothly? If it has substantial misfire that can lead to low readings,

Also make sure the harness for the O2 sensor is not grounded to the exhaust.

If this is all OK then the O2 sensor is almost certainly bad.