laserjet pro 200 MFP m276nw : 50.4 fuser error (with replaced fuser)

[pathey42]

Hello community,

after a fuser error 50.2 I replaced the fuser with a new one. The error switched to 50.4, which appears shortly after powering up. I have followed the information  from the thread from DAN-Kov here: https://h30434.www3.hp.com/t5/Printing-Errors-or-Lights-Stuck-Print-Jobs/laserjet-pro-200-MFP-m276nw....

The photorelay on board RM1-8705 gives a voltage of 3.00V in obstructed state.

The second photorelay  on that board for the backside panel detection gives 3.30 V obstructed and 0.13V unobstructed. I am in doubt, if one sensor RPI-579 is the cause,

Does anybody know which voltages are expected at the different  test points of the board?

Best Regards

[Max3Aj]

@pathey42, Welcome to HP Support Community. 

Thank you for posting your query, I will be glad to help you. 
You're dealing with a 50.4 Fuser Error on the HP LaserJet Pro 200 MFP M276nw even after replacing the fuser, which suggests that the issue may not be with the fuser itself but with power delivery, a fuser detection circuit, or a sensor malfunction, such as one of the photointerrupters (RPI-579).

You tested the photointerrupters on DC Controller Board RM1-8705:

One gives 3.00V when blocked (likely fine).

The backside panel sensor gives 3.30V when blocked and 0.13V when unblocked — this behavior appears correct; most RPI-579 photointerrupters work with high logic in blocked state and low when clear.

So far, your voltages look within reasonable range, though 3.00V seems slightly low (typically 3.3V logic). This alone may not cause a 50.4 error unless there's unstable or fluctuating readings or poor solder joints.

Check fuser compatibility:

• Are you absolutely sure the replacement fuser is identical in voltage rating (110V vs 220V) and model-specific?

A mismatched fuser model will cause a 50.4 error.

Inspect the fuser connector:

• Ensure the connector pins on both the fuser and printer side are not bent, dirty, or misaligned.
• Also, check fuser thermistor and heating element connections.

Reinspect the RM1-8705 board:

• Closely inspect solder joints around the photointerrupters and relay.
• Look for signs of dry joints, overheating, or corrosion.

Check the AC Power Supply Board:

• If the AC power board has failed to deliver the correct power to the fuser, a 50.4 error can occur.
• You may want to probe power output from the low-voltage power supply to the fuser connector (requires caution).

DC Controller Board Fault:

• If the DC controller cannot detect or communicate with the fuser correctly, it will throw a 50.4.
• If you have access to a known working DC Controller Board (RM1-8705), consider swapping temporarily to test.

I hope this helps. 

Take care and have a good day. 

Please click “Accepted Solution” if you feel my post solved your issue, it will help others find the solution. Select "Yes" on the bottom left to say “Thanks” for helping! 

Max3Aj

HP Support 

[pathey42]

Thank you Max3Aj for the extensive help comment.

I have some information to add to my first post:

- The fuser ordered is the RM1-8781 220V. Is there any way to differentiate the 110V from the 220V one? Externally there is only a sticker with a barcode number.

- I removed the board so I could measure unubstructed sensor: 1.24V   (Obstructed 3.00V)

- The supply voltage from RM2-8097 to RM1-8705 is approx 29V AC. Is this the expected value?

Best Regards

[Max3Aj]

@pathey42, Thank you for your response. 

Fuser Model: RM1-8781 220V

RM1-8781 is the correct part number for the 220V region (commonly used in Europe, Asia, etc.).

The 110V version is RM1-8780.

Photointerrupter Voltage Readings (Sensor RPI-579):

• Obstructed: 3.00V
• Unobstructed: 1.24V

This is outside typical expected behavior.

Expected Logic Levels:

• Obstructed (beam blocked) → HIGH (typically ~3.3V)
• Unobstructed (beam clear) → LOW (typically < 0.5V, ideally < 0.2V)

So your 1.24V is suspiciously high for a LOW state. This could point to:

• A weak LED in the sensor
• A bad pull-down resistor or damaged transistor output
• Or a signal line with poor grounding or open connection

This sensor could absolutely be the cause of the fuser communication failure if it’s used in the fuser detection chain.

Supply Voltage: 29V AC from RM2-8097 to RM1-8705

Yes — ~29–30V AC is expected and normal for this line.
It’s used to drive the low-voltage power supply and trigger heating circuits in the fuser.

I hope this helps!

If my response resolves your issue, please click “Accepted Solution” to help others find the answer. Also, don’t forget to click the “Yes” button to say thanks!

Take care and have a great day.

Max3Aj

HP Support