The Volvo code for a 121 is Mass Air Flow sensor signal missing or faulty. That goes right back to bad contacts in the plug to the harness, or a bad/bent wire about 1in infront of the plug. Or, heaven forbid, a bad MAF. Dealers around here tend to replace the MAF before suspecting a dirty connection at the plug. I would clean the connection first, check for any 90degree bent wires, change the air filter, look for cracks in the air flow tubing, before replacing a costly MAF.
The 121 code is self resetting if the problem goes away, after two engine cycles.
Klaus
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98 V70Rawd(108Kmi), 95 854T(88K mi), 75 164E(173K mi)
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