I haven't replaced my o2 sensor yet, but now i'm getting 113, 157, and 172.
Could bad o2 case readout for maf to be bad, and what the heck does the open circut below -40F mean anyway?

 

113 indicates open circuit in air temp sensor, located in air tube after MAF, wire fell off or failed sensor? looks like water temp sender, critical for correct air/fuel ratio.

157 is low MAF voltage reading; contaminated, damaged or burned & open MAF heated element or sensor, may be intermittent.

correct air temp sensor, clean or replace MAF, clear codes and retest...O2 172 code lean may be related to other failures. if more than 100k per Bosch on 4 wire heated OBDII systems and 60k on OBDI pre '96 systems-replace O2 after other conditions corrected or may contaminate new O2 with overrich mix.

 

I've already cleaned and checked maf (i do this monthly).
everything it good on the maf. The sender unit could be bad, is there a way to check with resistance?

 

don't have any info on testing IAT

Intake Air Temperature Sensor

The Intake Air Temperature (IAT) Sensors (Figure 21) are thermistor devices in which resistance changes with temperature. The electrical resistance of a thermistor decreases as the temperature increases, and increases as the temperature decreases. The varying resistance affects the voltage drop across the sensor terminals and provides electrical signals to the PCM corresponding to temperature.

Thermistor-type sensors are considered passive sensors. A passive sensor is connected to a voltage divider network so that varying the resistance of the passive sensor causes a variation in total current flow.

Voltage that is dropped across a fixed resistor in a series with the sensor resistor determines the voltage signal at the PCM. This voltage signal is equal to the reference voltage minus the voltage drop across the fixed resistor.

The IAT provides air temperature information to the PCM. The PCM uses the air temperature information as correction factors in the calculation of fuel, spark and MAF.

The IAT sensor provides a quicker temperature change response time than the ECT sensor.

some auto parts stores have MAF testers for free, other choice is auto repair shop with engine analyzer that can run a MAF dynamic test with storage scope, about the same price as new MAF, will catch MAF's that are sluggish and no longer responding to fast WOT changes in air flow. MAF elements are like O2 sensors, become contaminated and are fragile-eaisly damaged, not always cleanable, service item like a fan belt.
sticking partially leaking PVC valves with high engine blowby are the worst killers of MAF's. search Aero forum for cure.

Mass Air Flow Sensor

The Mass Air Flow (MAF) Sensor uses a hot wire sensing element to measure the amount of air entering the engine. Air passing over the hot wire causes it to cool. This hot wire is maintained at 200°C (392°F) above ambient temperature as measured by a constant cold wire (Figure 23). If the hot wire electronic sensing element must be replaced, then the entire assembly must be replaced. Replacing only the element may change the air flow calibration.
The current required to maintain the temperature of the hot wire is proportional to the air mass flow. The MAF sensor then outputs an analog voltage signal to the PCM proportional to the intake air mass. The PCM calculates the required fuel injector pulsewidth in order to provide the desired air/fuel ratio (Figure 24). This input also used in determining EPC, shift and torque converter clutch scheduling.

 

I had the 172 code. It ended up being caused by a bad fuel pressure regulator.