The junction fot the O2 sensor signal to ECU is on the right side of the firewall. Test for rapid cycling in the 0.2 to 0.7 volt range.

For a more thorough workout, a test of O2 sensor/ECU interaction can be made by sending simulated Full RICH and Full LEAN O2 sensor signals to the ECU, and watching for appropriate responses from the O2 sensor. This is done "Open Loop", (i.e., with the O2 sensor signal unplugged), and with Voltmeter on the O2 sensor lead. The engine and O2 sensor should be fully warmed up for this test.

Full Rich Test: Apply a + voltage potential to the ECU Green wire. Use either the + terminal of a grounded 1.5 volt (Flashlight) battery, or hold the ECU (green wire) terminal with one hand and put the other hand on the Battery Positive terminal. The ECU should see this as a "Rich" O2 signal and set a very lean fuel mixture. The Idle should worsen (may even stall), and the O2 sensor should respond with a correspondingly low output (.1 to .2 volts).

Full Lean Test: Ground the Green wire going to the ECU. This simulated O2 "Lean" signal should result in very rich setting from the ECU . The RPMs should increase, and the O2 sensor output should rise to nearly a full 1.0 volt.

These tests are described in the the Bosch FI & Engine Management manual (p 6-22, "Rich stop" and "Lean Stop"), for Continuous Injection (CI) K-jet, but are equally applicable to LH injection.
--
Bruce Young
'93 940-NA (current) — 240s (one V8) — 140s — 122s — since '63.

I had a similar problem with a 89 740 with 85K miles; runs rough, light goes on, codes point to sensor, problem clears and then reoccurs. A dealer test on the sensor showed it was functioning OK but "lazy"...translate it sometimes doesn't work. Dealer price to replace is $300++; I am inclined to replace it myself. Check the Bosch web site; there is a detailed discussion of sensors there, including replacement instructions. Price for the part about $120 at FCP Groton. I am no genius at this sort of thing, and consider my most elaborate job to be replacing a water pump on a Chevy.

If it does turn out to be your O2 sensor, I do recommend you get the generic and splice it in. I have good results with mine using crimp connectors and covering with the heat shrink wrap tubing available at parts stores. It does get wet down there. They say not to solder them because these zirconia sensors "breather" through the wire insulation tube.

Oh, and yes, the white wires are for the heating element and it doesn't matter which way the current flows so the wires are interchangeable.
--
Andy in St. Paul, '91 745 Regina 198K mi, '89 244 141K, '87 245 RIP

Just because you got a 2-1-2 code doesn't mean you need to change the O2 sensor. Been down that route and it can get pricey. Check the O2 sensor out with a mulitmeter and see if the volts are between .1 and .9 staying close to .5 volts. It your sensor is giving proper voltage look else where. Check FAQ section.