According to my Bentley manual, the Engine Management for the '96 ~ '97 1.9L
engine has the O2 Sensors configured with BLK, YEL, BRN, & GRN wires. The
BLK wire is ground, and the ground on the front sensor is tied to the body
of the sensor, but on the rear sesnor the ground is isolated. (I suspect the
rear mount is not a qualified ground because it is part of the exhaust
system that is isolated from the block via a gasket and rust.) A problem
with this ground will affect the data that the sensor generates, it will not
affect the heaters.
In any case, the sensor wires are BLK & YEL on both. The BRN & GRN wires are
the heater wires. The pin outs are
This wire is tied to hot (12vDC).
This wire is Heater GND.
This wire is Sensor GND
This wire is the Sensor SIG
I can't suggest a static test, and you may need one because the wore colors
you reported are not the same as in my manual. Frankly, your engine isn't in
my manual either -- my book does not show a '96 318 ti. Assuming your ti is
the same as the 318, my source should be okay. (There are several references
that say, "except '97 318ti," but the O2 Sensors have not got that warning,
so they should be the same.)
My gut feeling is that you spliced incorrectly.