Ricoh Aficio 551 w/ sc366

Thermistor failure.

https://www.google.com/search?q=Ricoh+Aficio+MP-6500.+Error+message+SC+551

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John;; links below will help you solve the problem concerning sc551 error. PRESS THE HELPFUL BUTTON

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Ricoh Error Code SC 551 CopierMaps com

Error Codes Ricoh Aficio 470W Code SC551

Thread Aficio 2060 with SC551 codes

You just have a problem with colour PCU (SC 365 - magenta, SC366 - cyan). The main reason is that toner density is abnormal. Sometimes, when you replace toners, printer receive to much toner for a short time, and after that you have an abnormal toner density (higher). You can check in service mode (hold up and down arrows for 6-7 sec, release them and press ok button) following thing :
choose 2. engine
then with arrows choose 3. process
then choose Vt display,
for each color by default there should by somewhere arround 2.4V. If you see for some colors, for example 0.02V, then this is a problem(to much toner). In this case the only thing that you can do is to restart printer and try to print enough pages, filled with problem color. Usually you can print max 3 pages. Then restart printer and try another 3 pages. Print until the you will be able to print at least 4 pages continuously.
Good luck and remember that this is a technical problem and you should ask someone qualified for the job. I am sure that 80% of service engineers can't repair this issue. But I can.

This is a TD sensor output error.
Causes:
Defective connector
Decreasing toner density
Replace the Cyan PCU.

Definition- [B]
No encoder signal from the scanner motor
-Possible cause-
• Scanner motor connector defective
• Scanner motor defective
• MCU defective
• Scanner wire, timing belt, pulley out of position
• PSU defective
The screws of the pulley usually are cut that moves the scanner this causes that the car is crossed and that the sensor does not detect the home position. check and corect this defect.

I just came across this post, you've likely solved it by now; However it is a "Toner density" error related to your cyan photoconductor. It is a defective connector. Replacing the cyan photoconductor should solve problem.

cyan problem --check if cyan pcu is set properly or replace cuan pcu(drum)

sc541 is a fusing unit error code. Can you reset the code in service mode?

1. Mode(Yellow Button)
2. 107
3. Press STOP Button for 3 second (RED Button)
4. Copy Sp
5. 5810#
6. Excute
7. Power Off
8. Power On