What Is An Air/fuel Sensor? - Corolland: the Toyota Corolla Forums

Bad_dude

Hi Guys,

I have a 1999 Corolla. It's giving a code P0171 intermittently. So the mechanic told my wife that it's the air/fuel mixture sensor. Is that the same as the Oxygen sensor at location number 2? He told my wife the location is by the exhaust area. So I figure it's the 2nd O2 sensor. Another one told me it could be O2 sensor at location #1.

Thanks.

Both 1999 sensors on exhaust are oxygen sensors, and not air/fuel wideband sensors... Yours are interchangeable, except for different wires length.

P0171 signals a lean air/fuel mixture possibly from a vacuum leak on intake, and not a faulty upstream oxygen sensor on exhaust manifold... Downstream oxygen sensor past the catalytic converter(s) is only there to monitor catalyst efficiency.

Bad_dude

So does this car have the air/fuel wideband sensor? The mechanic said if we don't get this repaired, it will ruin other systems? How much truth and time do I have to get this repaired? Now the CEL is off and it's been off a 2 days now. So would this be?

Thanks.

Both 1999 sensors on exhaust are oxygen sensors, and not air/fuel wideband sensors... Yours are interchangeable, except for different wires length.
P0171 signals a lean air/fuel mixture possibly from a vacuum leak on intake, and not a faulty upstream oxygen sensor on exhaust manifold... Downstream oxygen sensor past the catalytic converter(s) is only there to monitor catalyst efficiency.

Your car has no Air/Fuel (wideband) sensor. It has oxygen sensors. One on intake manifold for air/fuel ratio feedback to ECU in closed loop operation, and one downstearm of catalytic converter(s) to monitoe catalytic efficiency.

To address P0171 lean air/fuel mixture code, you need to find the cause and rectify. It is usually from a vacuum leak on intake, somewhere between throttle body valve/idle air control valve and intake manifold gasket including PCV and vacuum hoses.

As dom mentioned, the wideband or AFR (air fuel or air fuel ratio sensors) are only on the 2005+ Corollas. The 8th gen - 1998-2002, all use conventional O2 sensors. Problem might be with the terminology - some mechanics incorrectly use terms to describe a part or diagnosis.

P0171 can sometimes come from an exhaust leak - you could be bleeding exhaust or siphoning fresh air before the upstream (#1) sensor before it can read it - make it read lean. The downstream O2 sensor (#2) sensor is mainly used for catalytic converter efficiency checks, doesn't really play too much with the air/fuel feedback loop.

Other things to check that could cause a P0171 - clogged or faulty injectors, stuck open PCV, excessive knocking, any number of vacuum leaks as mentioned by dom. Being intermittent also makes it a lot harder to diagnose. Have to catch it when it is happening to see if you can effect a fix.

Bad_dude

Thanks guys for the replies. Do you have a picture of what tubes to check for? This car is nothing like my 1990 Accord with a whole bunch of 8mm little tubes that I could replace.

Your car has no Air/Fuel (wideband) sensor. It has oxygen sensors. One on intake manifold for air/fuel ratio feedback to ECU in closed loop operation, and one downstearm of catalytic converter(s) to monitoe catalytic efficiency.
To address P0171 lean air/fuel mixture code, you need to find the cause and rectify. It is usually from a vacuum leak on intake, somewhere between throttle body valve/idle air control valve and intake manifold gasket including PCV and vacuum hoses.