A sudden ACOS rise usually has one of four causes: a competitor started bidding on your terms and pushed CPCs up, your conversion rate dropped, your campaign structure changed and budget shifted to weaker terms, or you lost the Buy Box. Check CPC and conversion rate separately — ACOS is a ratio, and knowing which side of it moved tells you which problem you have.
Split the ratio
If average CPC rose while conversion held steady, this is competitive pressure. Someone entered your auction, or a seasonal bidding surge began.
If CPC held steady while conversion fell, the problem is on your listing — price change, a lost Buy Box, a new negative review, or going out of stock on a variation.
The Buy Box check people skip
Losing the Buy Box suppresses conversion sharply while your ads keep spending, which shows up as a sudden unexplained ACOS spike. Confirm Buy Box percentage before rebuilding campaigns you did not break.
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