My 2001 Civic HX presented a check engine light with OBD 2 Codes P1166 and P1167. Runs normal with the exception of not idling down after acceleration or reving the engine. RPM's come down to 1200-1500 (normal running temp) for up to a second before returning to normanl RPM, approximately 750. Codes indicate primary O2 sensor. Deeper diagnostic did not reveal any other apparent problem and mechanic indicated the idle control valve suspect. I changed the sensor and cleaned the idle control valve, which did appear a little sticky. After leaving the battery disconected for a time, the check engine light came back on after a few seconds and the idle still hesitates to come back to normal. There are not other indications oather than a loss of 4 MPG. Help would be great before I jump into expensive parts. Thanks

My 2001 Civic HX presented a check engine light with OBD 2 Codes P1166 and P1167. Runs normal with the exception of not idling down after accelleration or reving the engine. RPM's come down to 1200-1500 (normal running temp) for up to a second before returning to normanl RPM, approximately 750. Codes indicate primary O2 sensor. Deeper diagnostic did not reveal any other apparent problem and mechanic indicated the idle control valve suspect. I changed the sensor and cleaned the idle control valve, which did appear a little sticky. After leaving the battery disconected for a time, the check engine light came back on after a few seconds and the idle still hesitates to come back to normal. There are not other indications oather than a loss of 4 MPG. Help would be grat before I jump into expensive parts. Thanks

Just noticed there are 3 O2 sensors. I replaced the one right in the midddle of the exhaust manifiold top front. There is another in front of the Cat and one behind. What's the difference between the 3?

The other two sensors measure the oxygen content of the exhaust gas stream entering and exiting the catalytic converter.

The other two sensors measure the oxygen content of the exhaust gas stream entering and exiting the catalytic converter.

OK so after I get a good Denso in the top front location (This is an HX which that sensor runs around $200) and the lead o2 sensor, this should fix the problem?

I would replace all of them with Denso oxygen sensors. I think this will solve the problem. Remember to reset ECM, after replacing the sensors.

I would replace all of them with Denso oxygen sensors. I think this will solve the problem. Remember to reset ECM, after replacing the sensors.

You were right. After I replaced the top front sensor with the correct (Civic HX) Denso sensor it cleared the CEL after ECM reset. I still have the idle concern. Likes to hover at 1200. Will come down to 750 after 30 seconds parked idle. AC does not change idle. Power steering while not moving elevates it to 1000. Reving the engine shows the rpms to come down, flutter at 1200 and then calm to 750. Coming to a stop sign it takes longer to calm, if at all. Cleaned IAC, but notice the screws that mount the electrical to the mechanical have significant space for what looks like adjustment (clockwise or counter clockwise). Any help with making it idle down properly would be helpful.

BTW - My 1991 GMC K2500 has the identical idle problem and a new IAC did not fix...

You may need to adjust the throttle cable. Also, ensure the throttle cable has not come out from one of the mounts near the master cylinder reservoir. I have seen this happen before.

Didn't notice if you had cleaned the throttle plate, the IACV is controlled by the PCM of course but there are several items that control the idle input so the output can be adjusted by the IACV.

Honda even knows when it is in Park or in Gear, the Electrical Load Detector (ELD) even comes into play and so does the VSS, the idle control portion of the PCM knows when the car is moving via the VSS and controls the IACV.

Can you hold revs up in Park, let off the gas pedal and see what the idle does. Then brake torque it, put it in drive, hold the brake with one foot and rev it up for a few seconds then let off the gas pedal and see what the idle does, it may act as though you were driving. It may very well be the IACV sticking for that 30 seconds, does the idle drop quickly or come down slowly once it decides to finally drop. There are many things that can cause this, a scanner test drive to see what all the scan data is doing will be best, a scanner should show IACV command & IACV position, maybe the command is saying go down while the position is sticking?? Maybe the throttle plate is sticking and then releasing, check TPS voltage for that one.