miata p1135 oxygen sensor problem

Hi,
99 miata here.
I have been having a problem with check engine light coming on..
Code reads p1135, Heated O2 sensor bank 1 sensor input low- it is the pre-cat heater element, as I have been told by the dealer. 1)I changed the sensor with bosch universal and 3days later the same code came back. 2)Went and exchanged for another one, same after 3 days or so..
3)I went to the wrecking yard found one, off of a 99, installed it same...... 4) Ordered one from internet made for my car, made by walker. Installed it and again the same thing.

I measured the resistance across the heater element of the O2 sensor and I got 16 ohms. I went and checked for resistance in the back (after the cat) sensor and I got approximately the same value. I heard normal value for O2 sensors are 4-7 ohms (http://www.asashop.org/autoinc/dec2002/mech.cfm).

What is the normal value form miata (California emissioned vehicle). Any of you out there have similar problem as I?
Your help would be much appreciated.

I have the same problem. Error code P1135. Dealer told me front O2 sensor needs replacement ($350 dealer quote). Bought sensor over Internet and installed yesterday. Did reset check engine light (disconnect battery). Light came on after about 10 miles driving.
Any suggestions???? Thanks

I have the same problem but haven't replaced anything yet. Have you come to any conclusions yet. And how about checking your gas cap?

I have a 2001 Miata. Where can I find the directions on how to replace the O2 sensor? Does it come with the part? I bought an NKT front O2 sensor which is in a heat sealed containner. Looks like there is a sheet of paper with directions on it. I have not opened it yet. I was going to do it when I read somewhere that I'd need a special tool (crow-foot wrench?). I also read that there are 2 sensors (front and back). I only bought the front one. Am I supposed to replace both at the same time? Thanks, Hieu