Originally Posted by Krommeweg

Hey Folks

I'm chasing a P0171 and in the process of testing sensors. Speaking as more of an idiot than an electrician. The car is running smoothly, just a bit down on power and throwing the too lean code.

I've cleaned the MAF and the entire intake thoroughly, found no vacuum leaks, new air filter, plugs and wires, new fuel filter, still too lean. All OEM / Denso parts. Electrical grounds and fuses are in good shape. This is a "federal emissions" vehicle, so one cat, sensor on either side.

My multi-meter has been giving me some readings pretty close to what Toyota has in the Factory Service Manual. I have been getting a few odd or incorrect readings as well. I have been surfing the "search" function on these forums and how-to videos as well, but I still have a few obvious questions.

Generally when using a multi-meter to read resistance, I have been setting the meter to 200 Ohm. Is that the correct setting?

Ambient temp during this process is about 30 degrees F.

Testing the fuel pump itself, the FSM says 0.2-0.3 at 68F. My multi-meter reads 1.1

A/F sensor (upstream from the cat) should be 0.8-1.4 at 68F. Mine reads 12.1

O2 (downstream from cat) and MAF sensors appear within range.

These numbers lead me to believe my A/F sensor is bad (or is actually an O2 sensor and I didn't look at the part numbers close enough), so ordering another Denso replacement now. Does the reading I get from the Fuel Pump seem acceptable?

While testing the MAF sensor the FSM tells you to check resistance on the harness side of the plug to the MAF sensor. It looks like you're supposed to just shove the multi-meter probes into the back of the plug through the squishy insulation around the wire... This seems like a bad idea, am I wrong?

Thanks for reading, thanks for your patience with the obvious questions.