Chevrolet Repair: idle too high, throttle position sensor, obdii scanner

Question
I have a 97 Blazer with a 4.3 liter motor. While driving home from work the car started sputtering and pinging a little, and the idle increased by about 500 rpm. After a few more miles the "service engine" light came on. For a few dollars the local mechanic hooked up his obdII scanner and the computer dropped the code for idle control valve. I replaced the valve with a dealer part and there was no improvement. Idle still too high and some pinging. Mechanic hooked up scanner again and he read three codes, idle control valve twice, and throttle position sensor.
I hooked up a different scanner for verification. It read one code: throttle position sensor. Replaced TPS and once again no improvement. The check engine light "did" turn off but the idle is still about 1400 rpm in N and 950 rpm in Drive. The scanner does not show any codes at this time. There are no obvious vacuum leaks. Battery, plugs, wires and filters are new, and this car has always been well maintained. 160k miles. Nobody has stuck their fingers under the hood to adjust anything. The A/C compressor works fine and doesn't change anything whether it's on or off. I am out of ideas. I would appreciate any direction or advice. Thank you in advance. Ed..

Answer
Hi Ed... check the idle air control... use a good air intake cleaner    to clean it...if that doesn't help check the cat converter and see if it's plugged...also check to see if EGR is working...if egr not working it will plug up converter and idle will bump up to compensate for the back pressure....hope this helps