A 50 service error can be either a temporary error or a permanent problem requiring service.

Temporary Error: Power off for 20 minutes to allow the capacitor on the DC controller to fully discharge. If you don't allow this capacitor to discharge, you will still get a Error 50 even if you have fixed the problem.  This may clear the error - either temporarily or permanently. If it clears the error it was most likely caused by a low power problem  i.e. low line voltage causing the power supply circuits to drop out of regulation. Low/unstable power (low line voltage, brownout condition).  If it doesn't clear the error you may have one of the problems listed below

The HP LaserJet II/IID/III/IIID, Apple LaserWriter II series and many other Canon SX engine based  printers have two common causes of the Service Error 50, a failure of the Triac in the AC power block assembly which requires rebuilding or replacement of the assembly, and a failure of the 14 tooth gear on the fuser assembly causing the roller to either become jammed or not turn at all due to a broken tooth on the gear. Other possible causes are the halogen heater lamp, the thermal fuse or the thermistor. The halogen heater lamp may be burned out, it should have less than 5 ohms of resistance between the two pins of the connector on the right end of the fuser assembly (you will need to remove the fuser assembly from the printer to access this connector).  If it measures as an open circuit the lamp or thermal fuse is burned out (these parts are connected in series).  If the error can be reset by turning the machine off for 20 minutes, it is most likely the AC power block.  This condition will only worsen until the error 50 is constant.  If the lamp begins to heat (you can feel the roller, but be very careful it's HOT ) its not the lamp or the thermal fuse and is most likely the triac (can replace with NTE 5608 or an NTE 5645) failing when current is drawn by the halogen heater lamp. Simply replacing the Triac is not recomended, there are two resistors and a solid state switch which should also be replaced. We have also seen the capacitor designated as C128 on the upper board cause problems. This 22 uf 25VDC electrolytic capacitor can leak. You can often verify the AC power block as the problem by opening the rear door of the printer and observing the heater lamp glow when turning the printer on - a dark room is best for this test.  Another clue can be a "clicking" sound just prior to the error 50 appearing.

When replacing the triac, you often need to replace the solid state relay and possibly two resistors.  It can be a labor intensive task requiring soldering skills (you don't want to peel those pads!).

 Many times, although not resulting in an error 50, the upper fuser roller simply becomes worn, the teflon coating begins peeling and a line can result on the printed page. You can check for improper fusing in the area around the line by running your fingernail across the print. It should not rub off! In these cases, the entire assembly must be replaced. It is not cost effective to replace only the defective part - upper roller, single gear, etc. It's like tearing your engine apart to replace a single piston ring.

When you purchase a remanufactured fuser assembly, be sure you know what parts have been replaced and if OEM or after market parts have been used. Like anything else, there are both acceptable and exceptional parts available. We supply fusers which have been remanufactured using only the highest quality parts available and we replace little things like the white rollers and the gear arm and separation claws. It's the same old story, you pretty much get what you pay for. All our remanufactured fuser assemblies carry a one year warranty.

On HP laserjet IIP/IIIP/Apple Personal LaserWriter printers, a Service Error 50 is most likely due to defective heater lamp. The halogen lamp burns out and can be replaced quickly. You can check this by observing the coloring of the lamp, it should be clear with the filament visible. To observe the lamp, you must first remove the fuser assembly from the printer.  The assembly is located on the door of the printer and is the black felt/fuzz covered assembly with the "Warning High Temperature" imprint.  To remove this assembly, remove the five brass colored screws.  Slightly flex the housing and lift straight up and out of the printer.  On the left end of the assembly you will see the ceramic lamp end with a white wire connected to a terminal.  Remove the screw securing the lug to the terminal. Next remove the clip securing the lamp. On the right side of the fuser is a set screw - look in the area of the cover latch.  Loosen this screw. You may now slide the lamp out of the fuser assembly.  Do not touch the glass portion with your fingers as finger oils will create hot spots and cause premature failures.  If the lamp is discolored it's developed a leak at the glass to ceramic seal and the filament has oxidized and is burned out. Occasionally, the power supply is at fault, usually on older machines that were manufactured early in the product life cycle. A manufacturing problem causes premature failure of the power supplies. To identify if you have one of the suspected power supplies in you HP LJ IIP, first check the S/N. This problem applies to S/N's below 2951JXXXXX. If your S/N falls in the range, then look for a RED dot on the power supply in one of two locations. You will have to remove the covers to do this. The first location is just above the AC inlet either on the P/N label RG1-1782 or to the left of the label. The other location is on the side of the power supply on the P/N label. If you do not have the red dot, then this is probably a defective power supply causing the 50 Service error. We have replacement power supplies available.

On the HP Laserjet 4/4M/4+/4M+, our experience so far has shown the halogen lamp to be responsible for most error 50 problems. The thermal fuse and thermistor are also possible causes of a 50 Service error. You can remove the fuser assembly from the printer and measure the resistance of the lamp/thermal fuse circuit - it should be less than five ohms.  Measure between the two heavy gauge pins on the fuser assembly connector. If you measure an open circuit, the problem is most probably the lamp. Removal of the lamp, although not difficult, requires knowing where the snaps are located to remove the plastic covers. We carry this lamp in stock and can replace it in three quarters hour of labor.  We are working on instructions to include with the parts detailing the replacement procedure.  Please let us know if you would find this useful. 

On the HP LaserJet 4L/4ML, errors are indicated by a series of indicator lights.  This printer has four lights, Paper Jam, Paper Out, Data, and Online. 

If all four lights are on, press and hold the blue panel button to display a secondary light pattern.

An error 50 is indicated by the ready and error lights with the blue panel button pressed.
Turn the printer off for 15 minutes to see if the error clears.  If not, you will need to remove the fuser assembly to perform further checks.  This requires removal of the cover an unpleasant task and measurement of the fuser terminals.

The HP LaserJet 5P/6P indicates service errors in a similar manner to the 4L/4ML except there are 5 indicator lights.  If the first four lights, top to bottom, are lit, press and hold the GO and RESET buttons.  If the paper jam light is lit, you have a Service Error 50.  Again, power down for 10-15 minutes or more to see if the error resets.  If not, you may have blown fuses on the DC controller board or a DC controller failure, but in most cases you will need to replace the fuser assembly.