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Faulty Air/Fuel Ratio Sensor and 4WD

I have a 2007 Toyota RAV4, automatic transmission with 4WD/AWD. About a week ago, the check engine, VSC, and 4WD warning lights all came on. The computer registered a faulty air/fuel ratio sensor. The dealer said that it won’t hurt the car to drive it while they order a new sensor, but I won’t have 4WD.

I understand that without the sensor I won’t be getting the optimal fuel mixture, but why does the computer shut down the 4WD system? It would be pretty handy right about now.


- Eric (Snowbound in Seattle)

Are the warning lights still on? Try resetting them by disconnecting the battery for a minute or two…Sometimes these problems fix themselves. I find it puzzling the dealer does not have an oxygen sensor for your vehicle in stock…Why this effects 4WD function is one of those mysteries of the automotive world…Sometimes engineers get to clever for their own good…

The engine computer NEVER “registers” a faulty component. It “registers” a faulty signal from a sensor, or other component. There is a difference.
If the dealer’s mechanics did no tests, or troubleshooting, they have made a wild guess, nothing more.
As a wild guess, I would suspect the VSS (Vehicle Speed Sensor signal) is faulty.
The VSS, Traction Control System, and ABS are interdependent.
There are checks to determine if the VSS, itself, is faulty, or if it’s caused by something else, such as wiring, or, even, a mistaken computer.

It’s probably in some sort of “limp home” mode right now. The way the 4wd system on this car works is that it automatically sends power to the rear wheels when it senses slippery conditions, but if it were to power the rear wheels on dry pavement it could cause serious damage to the transmission. So probably with any computer fault that could possibly confuse the computer it’s better safe than sorry and it just disables the 4wd altogether.

I would guess you’re right about the “limp home” mode. I sure wish there was an override though. Thanks.

It makes perfect sense with respect to the design of the Toyota AWD system. Basically your AWD is an extension of the traction control/VSC(stability) system. If it senses slippage it clicks on brakes to activate all those systems and removes power to the engine. It needs that particular sensor to remove power to the engine and cannot with it in a fault mode.

Your AWD is basically all computer controlled for better or worse. Thankfully you have an override for 4wd under 25mph which may work. My sister uses it a lot in more extreme conditions with hers as it does not get too far in deep snow otherwise.