'89 240 wagon - wifemobile
LH2.4 (auto trans)

Should I get someone to replace the oxygen sensor?

Car got about 20.5 mpg at the last fillup, those miles driven after doing a tuneup. Prior fillup was worse, about 18 or 19 mpg. Bosch cap + rotor, NGK plugs. This car usually gets more like 25 mpg. Tires are at 32 psi (wifemobile). I just now brought them up to 35.

Oxygen sensor test results, today:
Oscillates between .07V and .7V.
Bentley says it should oscillate between .4V and 1.0V so this seems kinda low.
Takes about 3 seconds per cycle. That is, I can start counting when I see .7V, watch it drop, and my count gets to 3 seconds when it comes back up to .7V again.

No check engine light. However:
Retrieved OBD code of 2-3-1 today which means (according to 700-900 FAQ):
Fuel trim (lambda control) too lean or too rich at part load. If too lean: air leaks, low fuel pressure, bad sensor. OR: Fuel trim (lambda control) too rich at part load: Oxygen Sensor.

I don't hear any vacuum leaks. When I pulled off one hose the oxy sensor voltage dropped but did not come back up. Only came up when I partially blocked off the open vacuum nipple (the one for flame trap's small hose at center of manifold "U").

What do you think? Time for an oxy sensor? I already have a list of low-cost oxy sensor part numbers that can be spliced into the car, so that discussion is taken care of.

Thanks, guys!
Practically everything I know about 240s, I learned here on BB.
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Sven: '89 245 NA, 951 ECU, expanded air dam, forward belly pan reaches oem belly pan, airbox heater upgraded, E-fan, 205/65-15 at 50 psi, IPD sways, no a/c-p/s belt, E-Codes, amber front corner reflectors, aero front face, quad horns, tach, small clock.