A test of O2 sensor/ECU interaction can be made by sending simulated Full Lean and Full Rich 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 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 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.

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Bruce Young
'93 940-NA (current) — 240s (one V8) — 140s — 122s — since '63.