I just added a 94 855 turbo ($750) to the stable that wouldn't start or idle correctly. Immediately pulled A2 codes and got 121 for MAF. Replaced MAF and cleaned elec terminals, code has not reappeared. Plugs were black and I replaced them. Now car starts and idles, but the temperature gauge will randomly drop to cold (and recover) from the normal 3 o'clock warmed up position. When it fluctuates, the idle also "stumbles" and as soon as gauge returns to 3 o'clock, it idles fine. I understand (from search) that the stumble is due to enriching the fuel because the computer thinks it is cold. (Thanks Klaus) I'm not getting an ECT code.
Secondly, A2 DTC now indicates rear oxygen sensor faults: 522 and 153. I cleared both and they recurred 2x.

Questions:
1) Does the dash gauge fluctuation necessarily mean the ECT is bad and needs replaced, or is it at all likely the electrical connector is very crusted (it was late last night and I have not checked yet) like the MAF was ?
2) Since it was running so rich, should I ignore the 522/153 codes until I get the car on the road and really heat it up to see if that cleans up the O2 sensor ?

Thanks in advance, Al
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91 745t, 95 855t, 90 745NA (B234 not running yet), 94 854 parts car