The differential for California-grade blendstock dropped 15.5 cents/gal on Wednesday after US data showed an 818,000-barrel gasoline build on the West Coast.
The move left the differential at a discount to NYMEX RBOB for the first time in nearly four months.
Platts assessed Los Angeles CARBOB at NYMEX August RBOB minus 5.25 cents/gal, down from the Tuesday assessment of RBOB plus 10.25 cents/gal. The blendstock was heard bid for July as low as NYMEX August RBOB minus 10 cents/gal and offered at minus 5 cents/gal late Wednesday morning PDT.
August Los Angeles CARBOB, not part of the Platts assessment, was heard traded at NYMEX September RBOB plus 3.50 cents/gal.
Traders said the market reacted to the US Energy Information Administration report showing a 2.9% gain in West Coast gasoline stocks in the week ended July 12.
Wednesday's federal report also showed 476,000 barrels of gasoline imports to the West Coast, tied for the second-highest total of any week this year. The week of June ended 21 had the same amount of imports, compared with 581,000 in the week ending May 24.
The last time Platts assessed the Los Angeles CARBOB differential at a discount to NYMEX RBOB was March 27, at minus 1.25 cents/gal.
