How to fix “an unknown error occurred” in HP 2135 all-in-one printer is not something that crops up often. When it comes to finding the answer for how to fix “an unknown error occurred” in HP 2135 all-in-one printer, experts recommend that the event log is inspected carefully for any error messages or signs. You should check the Applications & Services-Microsoft-Windows-Print Service and if there is nothing that you can view in the Admin log option, then you should get the Operational log enabled first prior to attempting a printing job.

If you are still thinking how to fix “an unknown error occurred” in HP 2135 all-in-one printer, on the basis of details provided by experts, this issue may take place when a document is being scanned by the device in question. This is a commonly used printer model and hence the unknown error occurred alert may come up in case there is an issue with the movement/functioning of the scanning platen. Some of the common reasons behind this error include a failing motor for platen actuation or even the platen light. Another reason could be failures of transfer of data to the memory buffer from the scanning head.  

In all these scenarios, the printer is usually perceived as having already deteriorated considerably. Yet, you can always reach out to HP Support for getting the problem fixed or if you still have your product warranty in place. You should consider checking the cartridges and other components carefully before switching off the printer. Then unplug all cables and the power cord from its wall socket before you plug them back in after about a minute or two. Once these are plugged in again, you should then get the printer restarted. In some cases, this may solve the problem altogether. Otherwise, you will require servicing/repairs conducted by authorized HP Support personnel.