P1130 Code ... MAF or O2 Sensor ?

My '01 PreRunner (2.7L Auto) just threw out P0171, P1130 & P1135.

I pulled the MAF and it looked brand new, but I sprayed the sensor element down with CRC MAF Cleaner for good measure anyhow and erased the codes.

Went for a drive to get some Liquid Wrench to start softening up the O2 Sensor threads and it returned the P1130 code.

A new O2 sensor costs $175 (Bosch or Denso) and a new MAF is $110 (AuotZone house brand) and even though the MAF is cheaper, I'm thinking the O2 is the one more likely to fail. So that will probably be my first shot at resolution if the codes return.

I read that you can measure the voltage output of the MAF to see if it is working properly, but don't know the procedure or what voltage to look for.

Thoughts ?