HP Printer Error Code 85 — What It Means & How to Fix It

What Does This Error Mean?

Error 85 on an HP printer is a MIO (Modular Input/Output) interface error. The MIO slot is where optional accessories like network cards or external print servers connect. The printer cannot communicate with the device installed in the MIO slot.

Affected Models

HP LaserJet 4000 Series
HP LaserJet 4050 Series
HP LaserJet 5000 Series
HP LaserJet 8000 Series
HP LaserJet 8100 Series

Common Causes

The MIO card or accessory in the MIO slot is not seated correctly
The MIO card itself is faulty or has failed
Dust or debris on the MIO slot contacts is preventing a proper connection
An incompatible MIO accessory was installed that is not supported by this printer model
The printer's MIO interface circuitry on the formatter board has developed a fault

How to Fix It

Turn off the printer and unplug the power cord. Locate the MIO slot on the back of the printer — it is a rectangular opening, usually covered by a metal plate when empty.

The MIO slot is different from the USB and parallel ports. It is typically labeled 'MIO' or 'EIO' on HP LaserJet printers.
Remove the MIO card or accessory from the slot. Inspect the gold contact strip on the card for dirt, corrosion, or damage.

Handle the MIO card by its edges. Avoid touching the gold contacts directly with your fingers.
Clean the contacts on the MIO card using a dry lint-free cloth or a pencil eraser, then wipe away any eraser residue. Reinsert the card firmly until it clicks into place.

The card must be fully seated. A partially inserted card will always cause communication errors.
If cleaning does not help, try removing the MIO card entirely and testing the printer without it. If the printer works without the card, the MIO card itself is faulty.

Running without the MIO card will disable whatever functionality it provided — such as network access if it is a network card.
If you need a replacement MIO card, check that the replacement card is compatible with your specific HP LaserJet model. Install it, reconnect power, and turn the printer on.

HP MIO cards are model-specific. An incompatible card will trigger the same error 85 even if the card itself is functional.

When to Call a Professional

If reseating or replacing the MIO card does not resolve error 85, the formatter board's MIO interface may be damaged. Formatter board replacement on these older LaserJet models typically costs $80 to $200. For older printers, weigh the repair cost against the value of the printer before proceeding.

Frequently Asked Questions

What is a MIO card and do I need one?

MIO stands for Modular Input/Output. It is an expansion slot on older HP LaserJet printers. MIO cards add features like network connectivity, external print servers, or additional ports. If you are not using any optional accessory in the MIO slot, you can install a blank cover plate and the error should not appear.

Can I use the printer while it shows error 85?

In some cases the printer may still function for local USB or parallel port printing even with error 85 showing. However, any feature provided by the MIO card — such as network printing — will not work. It is best to resolve the error before relying on the printer for important tasks.

My printer does not have anything in the MIO slot. Why is it showing error 85?

An empty MIO slot should not trigger error 85 on its own. If the slot is empty and you see this error, the formatter board's MIO circuitry may have developed a fault. Try installing a blank MIO cover plate to see if it clears the error. If not, the formatter board may need service.