I just failed the nox component on my 88/740 (about 280k km). Going around the bend on this.

Specifics

HC 3ppm vrs 71ppm limit
CO 0.02% vrs 0.4%
NOx 1090ppm vrs 805.

Obviously a really lean air/fuel mixture. I have replaced the cat (it was broken inside), cleaned throtle area completely using "FORT", replaced a few vaccum hoses that were not in the best condition. For the test I set the advance to 10degrees. Idle refuses to drop below 900rpm. I can't find an air leak anywhere, used a full can of carb cleaner spraying around and on all of the vaccum hoses and connections.

The engine does have blow by. Evidence of oil leaking around the dipstick and filler cap since owned, but not consuming much if any over the 7000km regular oil changes.

I have checked operational temperature of the motor at the coolent exit to the radiator and it seems to stabilize at 202 degrees f after a prolonged idle/1500rpm period. (I intend to lower the temp with a cooler thermostat).

O2 sensor wildly fluctuates between .25 volts and .77 volts during idle or 2000 rpm.

Before I replace the o2 sensor, I need to know if they can fail for improperly reading the oxygen content. ie, the numbers appear good, but is it possible that the sensor is not reading a bias, but still giving good results.

Interestingly, 2 years ago the car never got close to reading 1/2 the allowable NOx in the test (when I bought it).

I have replaced the sparkplugs also.

Does anyone know if a cheap NOx sensor exists? I'm thinking a probe that would connect to a multimeter for the do it yourselfer.

Any help would be greatly appreciated. I've already gone through the lanes 3 times without any real change.