Prices for diesel fuel and diesel exhaust fluid (DEF) – a critical component of the selective catalytic reduction (SCR) emission control systems on diesel-powered trucks – remain on an upward march in the U.S., with the national average cost of diesel fuel at the pump increasing for nine straight weeks now.

According to data compiled by the Energy Information Administration (EIA), the average U.S. price of diesel increased 3.8 cents this week to reach $4.127 per gallon, with the biggest jump registered on the West Coast (7.1 cents per gallon) while the smallest occurred in the Midwest (2.2 cents per gallon).

Meanwhile, pump prices for DEF reversed course between July and August, rising by seven cents to $2.77 per gallon, after falling by 28 cents, or 10%, to an average of $2.70 per gallon between June and July, according to data tracked by Integer Research.

Chris Goodfellow, an emissions analyst with Integer, noted that DEF prices increased or remained the same in all but three states in the U.S.: Michigan, Iowa and Connecticut.

Integer, which monitors DEF prices via its DEF Tracker service, added that the average price of bulk DEF deliveries across seven U.S. major metropolitan locations also moved upwards between July and August, though tote prices remained relatively level. The firm also noted that the average Canadian pump price for DEF remained level at $0.799 per litre between July and August, which roughly equates to $3.16 per gallon.

EIA also reported that the average price of gasoline across the U.S. also rose sharply this week by 6.7 cents to $3.843 per gallon. The Midwest witnessed the highest spike in gasoline prices (9.7 cents per gallon) while the West Coast registered the lowest increase (3.3 cents per gallon).