My other '88 wagon hasn't passed smog due to high HC. I measure 13.7V warm at the pin 3. I believe this means the controller isn't working. The car has a hard time starting when cold. I replaced O2 sensor, plugs, plug wires, rotor, cap, oil. The O2 sensor was a universal BOSCH with a sealed plug to splice in the orginal plugs. There are 2 white and 1 green. Besides these splices, what should I check next?

I get code #5, light stays on = Oxygen sensor signal illogical

Not sure how to measure voltage to the new O2 sensor. Should I try to expose a bare area to test while it's plugged in? Which 2 of the 3 wires? I've read it should be between 0 and 1V

I measure the O2 plugs from chassis (ECU) with the sensor disconnected.
13.63V between Brown and Black wire with white stripe
12.98V between Black wire with white stripe and Green wire
0.49v between Brown and Green wire

I'll swap a spare ECU to see if that helps...

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'88 300TE, 175k, black RENNTech 3.6L Recaro C's AMG 1 SOLD
'92 500E, 110k, Spruce green, stock SOLD
'94 E320 Cabriolet, 130k, E500 wheels, Emerald green SOLD
'94 E320 Cabriolet, 110k, black, stock, SOLD
'88 300TE, 229k, dark grey, SOLD
'90 300CE, 212k, white, new paint, SOLD
'91 300E, 209k, white, rebuilt head SOLD
'74 914-6, grey, 2.7L 325hp twin turbo Audi conversion

What is your duty cycle when you have the key in position 2?

Also, when you start the car from a cold start. (Engine cold) what is your duty cycle then?

Ignition Position 2: 70% for federal car, 85% for California, 100% means a fault has been detected.

From what I read I think that the oxygen sensor may not be correctly wired.

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2006 BMW M5 "Heidi" @ 109,000 miles
2005 MBZ C55 AMG "Lorelai" @ 165,000 miles
1991 MBZ 300E "Benzachino II" @ 165,000 miles
1990 MBZ 500SL "Shoshanna" @ 118,000 miles

(On the hunt for a good used M103 engine as of 6/10/23, PM me if you have one to sell!)