HP LaserJet Pro M375/M475 — 79 Service error Turn off then on
HP documents this M375/M475 79 Service variant as a condition that can be caused by an incompatible DIMM.
Error meaning
An incompatible DIMM may be installed.
This meaning is tied to the reviewed family/platform and subsystem below; do not reuse the number or wording by itself on a different printer family or protocol.
Applies to
LaserJet Pro 300/400 color MFP M375/M475 Series
- HP LaserJet Pro 300 color MFP M375
- HP LaserJet Pro 400 color MFP M475
Common formatting
These are formatting/lookup aliases for this reviewed condition, not separate error pages.
- 79 Service error Turn off then on
- 79-service
What usually causes it
- A memory DIMM that is not compatible with the printer is installed.
- A recently added or replaced memory module is not seated correctly or is unsupported.
- The error began after an accessory/memory change and continues even without a print job.
What to try
Relate the error to any recent memory change
If the problem began immediately after adding or replacing a DIMM, keep that accessory as the primary suspect.
Power off before handling optional memory
Follow HP's product procedure and normal electrostatic precautions. Do not access other internal electronics.
Remove the incompatible optional DIMM or reinstall the correct supported one
Use only memory specified for this product family. If no optional DIMM was ever added, do not invent one as the cause.
Restart and test at Ready
Confirm whether the printer starts normally without the suspect memory before resending print jobs.
After each corrective action
Retry the same operation that produced 79 Service error Turn off then on. If it now works, stop there rather than changing unrelated printer, network, supply, or service settings.
If the code returns
If 79 Service returns with no optional DIMM installed, or with a known-supported module correctly seated, record the exact startup behavior and escalate to HP rather than substituting unrelated memory.
When service may be needed
DIMM access is the limit of this reviewed path. Persistent 79 Service errors or uncertainty about installed memory should go to HP/service; do not open controller electronics beyond documented user-accessible memory.
Continue with guided troubleshooting
If the printer still is not working after the reviewed checks, carry this verified error context into the guided troubleshooter.
Related information
Verified source
Official HP family support document explicitly covers M375/M475 and provides supply and 79-series control-panel meanings/actions. Color variants are reviewed as variants, not separate SEO opportunities.