What is causing my Honda civic RPM to act crazy?

I need help determining what is causing and idle problem with my 99 honda civic ex coupe. So just recently about 2-3 weeks ago i took my car to get my thermostat and coolant sensor replaced. After I was giving it back, i noticed that the next day it was reving high at about 1,300-1900 then it would drop to normal (1000) after few mins. Then sometimes it drops close to 400rpm. This happens every time i'm at a stop. I was thinking about taking it back to the mechanic but i'm not because first, he charged me a lot, and second i don't have a good feeling about this guy anymore. Also, i got a check engine light come on like 2 weeks ago, but then it went away. Now it came back up, I'll try getting the codes tomorrow but as of now any ideas?

Air.

  • Am i crazy for wanting a supersport motorcycle? I'm 18 and just bought a Honda grom. Rode it for 3 days and I'm enjoying it. Took my msf course and everything. But after riding a friends gsxr 600 i want a supersport myself. Bikes came really natural to me and i still respect them (not the grom) I love adrenaline and the grom already bores me after 3 days and 64 miles. Is this even normal for a first time rider?
  • What's causing the swishing noise coming from my 2008 Honda Civic Coupe? I have a 2008 Honda Civic EX Coupe. Just installed new brakes and rotors throughout. They mentioned my calipers will need to be replaced in the next year, but they couldn't identify exactly the issue creating the swishing noise coming from the wheels.
  • What is causing my car not to have power? (honda civic)? I have a honda civic ex 1994 my car hasn't been wanting to start so I thought my battery went out so I went to go buy a brand new one and still nothing now I have no power at all in my car. So I checked the fuses under the hood i seen the 80A fuse was blown so I went to go buy a new one and as soon as I connected the battery back up the new fuse had blown again. Can someone please help me out what can the problem be?
  • What is causing my 2002 honda civic ex (Automatic) to barely move in drive and reverse? My daughter uses this car and was working great one day and then the trans just about quit working. Had to tow it. When in neutral I can hear a high pitch whining sound from trans. Is there any way to fix this. The engine light is also on.