Hi I have a 2001 Chevy Blazer that failed the local emissions test not because of codes but because the tests came back with the sensors not ready. I borrowed a friends plug in tester to read engine codes and received a P0101 and a P0135 the check engine light is not on right now but it does come on occasionally. Also the tester said that the Catalyst and Sec Air monitors where not ready One last bit of info is that I replaced a bad thermostat last winter with a non-OEM version the needle barely moved above minimum. Thinking this might be part of the problem I replaced it with a new OEM thermostat and the temperature is right back where it should be. Any help is greatly appreciated.

PO101 Basically this means that there is a problem with the Mass Air Flow (MAF) sensor or circuit. The PCM detects that the actual MAF sensor frequency signal is not within a predetermined range of the calculated MAF value for more than 4.0 seconds.
The simplest thing to do is to reset the code and see if it comes back. Then start with the cheapest, easiest repair procedures:

• Inspect for the following conditions:
  An incorrectly routed harness--Inspect the harness of the MAF sensor in order to verify that it is not routed too close to the following components:
  - The secondary ignition wires or coils
  - Any solenoids
  - Any relays
  - Any motors
• A low minimum air rate through the sensor bore may cause this DTC to set at idle or during deceleration. Inspect for any vacuum leaks downstream of the MAF sensor.
• A wide open throttle (WOT) acceleration from a stop should cause the MAF sensor g/s display on the scan tool to increase rapidly. This increase should be from 6-12 g/s at idle to 230 g/s or more at the time of the 1-2 shift. If the increase is not observed, inspect for a restriction in the induction system or the exhaust system.
• The barometric pressure (BARO) that is used in order to calculate the predicted MAF value is initially based on the MAP sensor at key ON.
• When the engine is running the MAP sensor value is continually updated near WOT. A skewed MAP sensor will cause the calculated MAF value to be inaccurate. The value shown for the MAP sensor display varies with the altitude. With the ignition ON and the engine OFF, 103 kPa is the approximate value near sea level. This value will decrease by approximately 3 kPa for every 305 meters (1,000 feet) of altitude.
• A high resistance on the ground circuit of the MAP sensor can cause this DTC to set.
• Any loss of vacuum to the MAP sensor can cause this DTC to set.