asked Feb 18, 2017 at 6:28pm
Hp HP LaserJet 4SI

Error 50

Hello All,

Our LJ 4Si has recently been acting up. We have used the old work horse reliably for years until last week. When we powered up the printer we smelled something burning and then got an error 50 on the screen. Tried clearing the error and nothing would help it so I figured the Fuser was dead. Ordered a new old stock fuser (off of eBay a new HP printer maintenance kit) and replaced the fuser. Old gal powered right up no issues. I had the printer running for about 30 minutes when I again smelled a burning smell. I immediately turned off the printer waited a bit and turned her back on. I first got a "Language Config" on the screen. I set the language to English and the printer started warming up and then nothing. I see a quick flash of all the lights and then error 50. Now it is likely that the old stock fuser was bad and dies on me but very unusual in that it lasted thirty minutes and some test prints before it died... So I am wondering if there could be some other issues causing this (i.e. a problem that is killing the fusers). Any advice on where to begin testing is much appreciated. Thank you.

A quick f/u: I took apart the original fuser and the lamp is intact so whatever the issue is, it is not the fuser lamp.

by NeedHelp on Feb 18, 2017 at 6:54pm Add comment

Had this happen to me onsite once. I replaced the fuser and was chatting with one of the office staff when I smelled a burning smell just like you. It was too late as the fuser had fried. It was a long time ago and I don't remember the model. I believe it was caused by the power supply. Could also be a DC controller not being able to see the temperature and keeping the fuser on. Looking at the lamp is not a certainty. You really have to put a meter across it and check the resistance.

by moe on Feb 18, 2017 at 8:35pm Add comment


Thanks for the reply. Well I took the new fuser out and looked at it. There are two rollers in this fuser: a hard upper one and a soft/spongy lower one. Well on the new fuser the lower spongy one has melted off and is melted on to the top one. So the new fuser is definitely fried as well. Now the question is, was this a bad fuser/old fuser were the material failed or is it a PSU problem like you suggested. The quick and dirty check of PSU according to the service manual is the fans and motor running. All three happen so I am guessing the correct voltages are being supplied. I will of course check the appropriate connection points with an ohm meter to be sure. However, what else in the PSU could be doing this? i.e. what else should I be looking at? Thanks!

by NeedHelp on Feb 18, 2017 at 10:29pm Add comment

The fuser itself can't cause the problem. It has sensors to monitor the temperature and reports to the DC controller which tells the AC power supply to turn on or off the voltage to the fuser. In this case the voltage stays on and melts the fuser. The 50 error can indicate either an overtemp or undertemp condition.

by moe on Feb 19, 2017 at 2:39pm Add comment


So I am thinking one of the following:

1. Fuser roller foam had rotted so even though temp did not exceed 210 it melted

2. Fuser thermoresistor was bad and did not send shut off signal to PSU or DC controller.

3. DC Controller bad and did not receive signal or did not send shut off signal

4. PSU is bad and it did not shut off even though it got signal to turn off.

I am going to test 1 and 2 by putting in a new fuser assembly - If it happens again then 1 and 2 can be ruled out.

I am not sure how to test for "3" and "4" though it may not matter. After I got the error the fuser does not warm up at all. I.E. it stays cold to the touch (which of course produces an error 50). So if it was a controller issue or PSU where the power was not shutting off I would think the fuser should still heat up.

by NeedHelp on Feb 19, 2017 at 7:10pm Add comment

You do realize that you'll just destroy another fuser? I don't know why the thermal fuse didn't blow and shut the fuser off. You can check it on the melted fuser by checking the resistance. If it blew, it should be open.

by moe on Feb 19, 2017 at 9:29pm Add comment

Yes, I realize I may destroy another fuser. I am not sure where I would find the thermal fuse on the melted fuser. The service manual unfortunately does not provide that level of detail. If you can point me to where to look I am happy to check. Thanks.

by NeedHelp on Feb 19, 2017 at 11:33pm Add comment

Ok. I found a very old post on this site that describes measuring resistances. The fuser has 5 pin connections setup as such:

a D

b E


a,b,c are small connectors and measuring the resistance across a and c gives ~245K ohms (which based on the old post seems to be accurate).

I can not measure any resistance across the two larger/beefier connectors D and E.

However, checking continuity across the lamp proves positive (i.e. there is a short and the lamp is intact per the visual inspection).

So what seems to be burnt out is the cylindrical object that is sitting right against the fuser roller. In fact this reads open on both fusers which explains why the fuser will not warm up (it sits inline with the power circuit to the lamp). So is this the thermo-protector switch (looks like pictures of bimetallic disc switches I found online)?

Per the service manual: "The main components of the fusing system are the fusing lamp
and its associated Fusing and Pressure Rollers, a Thermistor, Thermo-protector, and the AC
Controller/safety circuitry in the AC Power Module. The Thermo-protector is a switch designed
for over-temperature protection and opens when the temperature reaches 210° C. The Thermo-protector may not reset when the fuser cools."

I am guessing the thermoresistor is fine given the 245K ohm reading, and I know the lamp is fine given visual inspection and continuity testing so that leaves the thermo-protector switch being tripped and given that it is the issue on both fusers the problem is probably higher up in the DC controller or PSU. Any advice on how to proceed from here? Thanks!

by NeedHelp on Feb 20, 2017 at 12:35am Add comment

The round thing is the Thermo-protector. I remember being able to buy them a long time ago. I don't know why you are so determined to fix it. It's big and slow. Cartridges are hard to come by. No new parts available, only used ones. I don't really care for the newest models. They're built in China and really cheap and shoddily constructed. I prefer the 4200/4250 series. That's what we use in our office. Faster than the 4si and easy to work on. Also much smaller and lighter.

by moe on Feb 20, 2017 at 3:58pm Add comment

Replacing the Thermo-Protector is not necessarily the solution. It may get the old fuser going but since it was tripped on both the old and the new fuser something else must be wrong. Proba ly up in the PSU board w/ the thermal regulation system.

by NeedHelp on Feb 21, 2017 at 10:22am Add comment

It's great to know all of this :3 thank you for sharing !

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