Diesel engine manufacturers are asking a Sacramento judge to block stricter pollution standards set to take effect in one week.
They argue that state regulators reneged on a $1 billion 1998 legal settlement.
The California Air Resources Board says the pollution standards it adopted in December are needed because a voluntary program promoted by manufacturers wasn´t working.
The judge tentatively denied manufacturers´ request, but did not immediately issue a final order.
The air board says removing so-called “smog defeat” devices from California-registered trucks alone would trim air pollution the same as removing a million cars from California highways.