I thjough the lambda sensors simply output a voltage that is dependent on the amount of O2 in their operating atmosphere? Using that output volatge an ECU can compute the AFR. If that is so, then the ECU uses that voltage output to measure the affects of the changes the ECU makes to the AFR. in which case the ECU is determinng the AFR not the lambda sensor? That being the case AFR is determined by the programming of the ECU. Ideally one would chnage the settings in the ECU program, to something more like 13.7:1 .
Semantics I know, but it is shitty out and crap on the TV
Wide-band sensors and devives to "skew" their output are often used to fool ECU into thinking that the AFR is where the manufacturers want it whilst the actual value is where we wousd like it, for exmaple the Innovations LM-2 kit. These would be a good way of getting the mixture "just-so", but with the R12000GS the cost doubles because we have one in each downpipe :-(