I've been chasing a recurring P0171 code (too lean bank 1) for weeks and tested just about everything, as well as tried to clean the MAF sensor. I also replaced the front O2 sensor with a £55 OEM Denso (from Walker Products) as well as the air and fuel filters. I could list all the tests I have done but it would be quite a long list.

I finally thought to check the MAF sensor latency using a cheap OBDII interface to my iPhone. I guessed that if it was dirty or dying it might be sluggish.

Here are before and after replacement shots. I have dropped in a new Denso sensor (Toyota branded) which cost just over £30.

The difference is striking. The new sensor has a faster response than the RPM value. The old one (probably original from 2001) was smoothing out air flow changes (and acting like a low pass filter) as well as under reading.

I hope this is helpful to someone. I've now got no acceleration lag - but didn't know what I was missing until today.

OLD - sensor response is delayed and output reading seems slow. Response to throttle closing seems particularly slow.




NEW - sensor response is more rapid than RPM change