One problem related to check engine light on has been reported for the 2004 Volvo XC90. The most recently reported issues are listed below. Please also check out the statistics and reliability analysis of the 2004 Volvo XC90 based on all problems reported for the 2004 XC90.
Exiting driveway in reverse, upon switching to drive the dashboard lights started flickering and a jerk was felt in the transmission. Soon the dashboard gauges and the indicators went dead. Several error messages quickly "scrolled" (brake failure, airbag failure, abs failure, engine failure) then the check engine light came on and remained on. No power to dashboard controls (gauges dead, no lights/turn signals, no radio). Front windows work, but not rear. Vehicle drives as if in low gear. Unsafe to drive so had towed to dealer. They are running diagnostics so tbd on specific issue and cost to repair. Concerned as no recall and situation is so dangerous (what if I'd been on freeway!) Volvo should recall all the 2004 Xc90 as others report same defect.