I pulled the codes from my Landcruiser. It read P0170 Fuel Trim Malfunction Bank 1. This started shortly after the head gasket was replaced. The mechanic that did the HG replace pulled the codes and recommended to replace O2 Sensor (Front) which I did. It obviously didn't help.
1995 Toyota Landcruiser 4.5 L inline 6 cylinder Auto transmission (I think 3 speed)
Here's a little more background: A few months ago I had to replace the head gasket. It seems to be a typical maintenance item for 80 series landcruisers at about 100k miles. I had this done at reputable shop in the area. About 15 miles after it was done the CEL came on. I went back to the shop and they pulled the code. They said it was a bad front O2 sensor as indicated by a P0170 code. I replaced the O2 sensor myself (Denso) and the code came back about 20 miles later. I've cleared the code a few times in hope that it would go away....no luck. I checked the MAS and it is clean. I have not been able to check the resistance. All vacuum tube connections look good. I checked the O2 sensor installation and it looks good.