I have a 91 740 turbo with about 550 000 km.

It has a fair bit of blowby as shown by oily smoke puffing from the filler cap, hose from the oil trap or oil dip stick opening but not enough crankcase pressure to force the dipstick up, so I assume the PCV lines are venting the vapours properly. It runs and idles well with no engine fault codes, BUT I'm not sure that the ODB I system still works, despite the 1-1-1 when checked.

I have just replaced the CAT, O2 sensor (both aftermarket though), as well as air filter, spark plugs, oil & filter as well and also tried one of the fuel system cleaners added to the fuel tank.

Unfortunately, the 3rd emission test was a still a fail with the results as follows:

HC: result = 76 ppm(pass = 60)

CO: result = .12%(pass = .33)

NO: result = 252 ppm(pass = 675)

Is there another sensor that is possibly defective that the OBD is missing?

Other suggestions in getting this to pass?

Thanks.