During descent after a 3-hour flight, the crew smelled exhaust-type fumes passing through 9,000 feet.

The crew turned the temperature control to “full cold,” and the smell disappeared. When the aircraft was level at 4,000 feet on the downwind leg, the crew selected increased heat on the temperature controller.

The fumes returned after a very short period of time, and the cockpit started filling with smoke. The crew donned oxygen masks, deployed passenger oxygen masks, and dumped pressurization. After turning the temperature control to “full cold,” the cockpit remained clear of any smoke.

Technician discovered the defog blower assembly (P/N 5703-3) was bad. The FAA Service Difficulty Reporting Program data base revealed 10 reports on defog blower assembly failures on the Cessna Citations.

According to the submitter, the circuit breaker does not trip when the defog blower assembly fails.

Part total time unknown.