Desperate Straights Check Engine Light Lit, CEL CODES: P1172 P2A00, pointing to a primary O2 sensor failure and resulting rich fuel/air mixture. In ideal summer conditions, I was having trouble breaking 60mpg on any trip and filled two tanks under 56 mpg . Fixed by replacing the primary O2 sensor for a total of $157. Now, no CEL light, current tank sitting at 63mpg :Banane26: --here's how. First, let me thank member Mendel Leisk for his detailed valve adjustment thread that clearly explains how to remove the cowl cover and cowl, allowing easy access to the sensor (it may be possible to do this job without removing the cowl, if you can get your hand on the sensor's connector and trust yourself to remove it by feel only). The sensor sits in the exhaust manifold facing the fire wall, you can see it plainly once you remove the plastic spark plug cover. http://www.cleanmpg.com/forums/showthread.php?t=17946&highlight=valve Must PDF from Mendel's thread: http://www.bernardiparts.com/Images/Install/2006_CivicHyb_EngineBlockHeater_All30320.pdf This YouTube video is a great primer. Tip: you do not have to remove the O2 sensor receptacle from its stand, the first part of the video. http://www.youtube.com/watch?v=CxKsGXhOc7o PARTS Denso OEM O2 Sensor (ships with bronze anti-seize): $146.25 http://www.amazon.com/gp/product/B001F770UU/ref=oh_aui_detailpage_o01_s00?ie=UTF8&psc=1 WD-40 with straw Zip Ties (to replace worn plastic cowl cover fasteners) TOOLS 10 mm wrench and socket (disconnect the negative battery terminal and 5 of the 9 cowl bolts) Electrical Tape (place roll over the negative battery post) 12mm socket (4 of the 9 cowl bolts) Socket Extensions – to reach sensor and bolts. 3/8 Socket Drive (I needed a ¼ inch for my 10mm socket, as well) Powerbuilt 22mm O2 Sensor Socket: 8.33 http://www.amazon.com/Powerbuilt-64...cp_1_ZVJQ?ie=UTF8&refRID=1CXZYMX340XAHHPCT308 Radio Security Code Directions 1. Disconnect Negative Battery Terminal. 2. Place Tape Roll Over Battery Post. 3. Follow directions in the above PDF and remove plastic cowl cover. You can pivot the cover to the left and avoid disconnecting the washer fluid line. 4. Remove cowl by removing all cowl bolts except the center bolt that inserts horizontally. It is possible to loosen this and then slide the cowl out, there is a notch in the cowl not a hole. Makes it easier to get back in, too. 5. Spray base of O2 sensor with WD-40 or equivalent to loosen. 6. Wait a few minutes. 7. Break O2 sensor free with socket and ratchet (very little effort in my case). 8. Unscrew sensor by hand. 9. Follow electrical lead by had to the connector...note the retaining clip the middle of the lead is fastened to, you will have to re-clip the new sensor. 10. The connector is located in a bank of three electrical leads and fasteners, it is the one in the middle. 11. Remove the connector by depressing the plastic lever (clip) on top...see picture below. Again, you do not have to remove the male connector from the stand (the part that runs to the engine computer). 12. Dab bronze anti-seize on the new sensor's threads. 13. Hand tighten the new sensor. 14. Firmly, but not with excessive force use socket to tighten sensor-- ½ turn?? 15. Snap connector into male receptacle and stand. 16. Fasten the middle of lead to the clip in 9. 17. Reinstall cowl. 18. Take a breather, installing plastic cowl cover flush takes patience and a cool head. 19. Replace plastic cowl cover. 20. Reattach negative battery cable. 21. Turn ignition to ACC...enter radio code. 22. Start car...hopefully no CEL, huge MPG.