Once the fuser assembly has been removed from the printer, inspect the upper and lower rollers for damage, check the gears for excessive wear and broken teeth. If the gears look worn you may also want to look at the fuser drive gear inside the printer. Once the fuser has cooled to room temperature you can test the heater lamp/s and thermistor with an ohmmeter as described below. In most cases you when the fuser fails you will read opens so the actual readings are not that critical. Always use caution when examining the fuser - the surfaces may be hot and can cause burns.
HP LaserJet 4000
Remove the fuser from the printer and check the heating element for opens across the two larger pins on the right it should read 13-15 ohms. Also check the two small pins on the bottom right, they should read between 200k-500k ohms. Take all readings after the fuser has cooled to room temperature. Examine the lower rubber roller and teflon sleeve for damage.
HP LaserJet 4100
Remove from printer then using the Ref point check the three points on the heating elements mainly for continuity. When testing the thermsitors on the small pins be careful not to bend pins they snap into place. Take all readings after the fuser has cooled to room temperature. Examine the lower rubber roller and teflon sleeve for damage.
HP LaserJet 4200 / 4250 / 4350
Remove the fuser from the printer and check both lamps with an ohmmeter. The readings are not that critical since most of the time if either of the lamps go bad they will read open. Also check the two small pins on the bottom right, being careful not bend the pins. Take all readings after the fuser has cooled to room temperature.
HP 4300
Remove the fuser from the printer and check both lamps with an ohmmeter. The readings are not that critical since most of the time if either of the lamps go bad they will read open. Also check the two small pins on the bottom right, being careful not bend the pins. Take all readings after the fuser has cooled to room temperature.
HP LaserJet 4 / 4+ / 5
Remove the two screws located on the metal frame that hold the fuser in place. Lift slightly and pull straight out of the printer. Take all measurements once the fuser has cooled to room temperature. You must power off the printer for 20 minutes if a 50 service error has occured.
HP 4V
Open the top cover and remove the four screws holding the fuser in the printer. Pull straight up and lift from printer. Make sure the clear plastic spacers under the fuser metal frame do not come out with the fuser. Remove the plastic cover to expose the lamp screw picture on the left. Using this point as reference check the readings on both sides of the connector pictured on the right. Examine the gears and rollers.
HP LaserJet II / III
Open the top cover and remove the four screws holding the fuser in the printer. Pull straight up and lift from printer. Make sure the clear plastic spacers under the fuser metal frame do not come out with the fuser. Remove the plastic cover to expose the lamp screw picture on the left. Using this point as reference check the readings on both sides of the connector pictured on the right. Examine the gears and rollers.
HP IIISi / 4Si
HP LaserJet 5000
Check the heating element on the larger pins for opens using the Ref Pt. Check the thermistor for a reading in the Mohms. Look at the teflon sleeve and lower roller for damage. All readings should be taken when fuser is at room temperature
HP 2200
When testing place meter probes where wires meet plug. Examine the condition of the lower pressure roller and teflon sleeve. Take all readings when fuser has cooled to room temperature.
