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Code 21 4Afe Avensis

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Hi all,

I have an 2000 Avensis 1.6. The engine light came on a few days ago so I jumped pins 4 and 13 on the diagnostic socket and it gave error code 21, which is the lean mixture sensor. I've close to 160,000 miles on the original sensor so that's not too bad!

As I understand it there's a heating and an oxygen circuit in the four-wire sensor employed in this model, and a failure of either would cause the engine light to come on. How would I know which circuit is causing the malfunction? If the heating circuit fails and the sensor circuit is okay then is this a big problem? Is it possible to clean/repair the sensor?

Has anyone replaced their sensor with an aftermarket one? Where is the best place to purchase a replacement sensor?



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Hi Ed

Have you considered cleaning the MAF sensor first with Isopropyl alcohol and a cotton bud? (Be very gentle with it)

this is quick and easy and may solve your problem

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if i remember correctly, on a 4afe in the carina and Avensis, it was a Lean burn sensor and you are correct it is the heating element.

For your info, the sensor will still be functioning correctly and making fine adjustments to the AFR.

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  • 4 months later...

Just to come back to this: I put the car through the NCT (after warming it up very comprehensively :P) and the emissions were well within the allowed range with lambda=1 and MPG is holding steady at around 38 (mostly town speed+some motorway) so no need for a new sensor!

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I had lean burn sensor failure and the car ran fine. On the motorway, I noticed a slight change throttle feel, when the EML turned off, then come back on later. The fuel consumption hardly affected.

There is a default setting which keeps the car within limits.

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