1999 discovery series one with 109,000 miles. Still struggling with poor fuel economy, and occasional cel. (code 1187) last time, about every 100 miles or so. Took car to land rover dealer thinking they had better diagnostic skills than me - $300 mistake. Mpg at about 8 mpg. New wires, cats, o2 sensors, plugs, wires, etc., etc. thinking o2 sensor was at fault I started monitoring live o2 data while driving. Thought I had found problem when bank one upstream sensor went to 0 volts, but it came right back to voltage range I would expect from misc. data gleamed from forums and has not done that again. It starts at about 1 volt and will dip down to 0.30 volt under heavy load. Fuel trim dips as well. However, I notice upstream bank 2 sensor stays at no lower than 0.9 volt and fuel trim stays at what I assume is max positive of +24.2 a majority of the time. Should I assume bank 2 upstream is bad because voltage does not fluctuate? From what I understand bank 2 would be passenger side on north american vehicle. Someone also mentioned idle controll stepper motor could be dirty or malfunctioning. I have not checked this yet, but truck idles fine. Also thought possible malfunction of coolant temp sensor, but it appears to function correctly from scanner data as well.

Have replacement 02 sensor to install. I am using ntk 25044, and this one is only about 6 mos. old with 1000 miles on it.

Any thoughts on what else to look at other than o2?

 

My scanner also shows open or closed loop, I would suspect your O2 issues are keeping you in open loop, and the bottom of the mpg scale. O2s should be changing voltage, any one that is static should be suspect. 1187 is heater circuit open for one of the upstream O2s. Maybe connector issues. In my signature you will find the link to the post with the GEMS ECU manual, tells you how the sensors work and what makes each code. P1187 is shown on page 47.

 

I will switch out the upstream sensor that isn't changing. I did also notice it was switching from open to closed loop though. Fuel trim is what looked off between the two sides though. Passenger side stays maxed at +24.2 short term most of the time. Maybe that is issue with fuel Econ. Thanks for help

 

Just wondering, but why would dealership service department not pick up on the o2 voltage difference? They claimed they test drove on numerous occasions, but I suppose they were not looking at data while driving and it had not reset cel until I drove it home

 

got to o2 sensor today. removed passenger side upstream. found two wires unhooked from connector, and for the life of me after about 2 hours it is impossible to get to. i have been unsuccessful in adding a pigtail on the four wires - why did they leave them in a place so hard to get to? (over the back of engine on top of transmission housing) anyway, i want to tap into main harness and extend plug ins so they will be easier to access and to assure they are tightly connected. my question is - i have looked to match the colored wires from the passenger side upstream o2 sensor. however, when i opened the main wiring harness i cannot find matches to all the colors and stripes present on the 4 wires to this o2 sensor. can someone tell me exactly where to look in the downloadable data to find the wiring diagram that correlates wire colors to connections to sensors?? as mentioned i have a '99 disco series one. i have looked through some of the downloadable rave data but have yet to locate a diagram that goes by wire color to function.