HP Printer Error 73 — What It Means & How to Fix It

What Does This Error Mean?

Error 73 on an HP LaserJet printer means a problem was detected with an optional I/O card installed in the printer's EIO (Enhanced I/O) slot. EIO cards are used to add features like network cards, extra memory, or Bluetooth to older LaserJets. The card may be failing, incorrectly installed, or incompatible. Removing and reseating the EIO card — or removing it entirely — usually resolves the error. The printer can still operate for direct USB or parallel port printing without the EIO card.

Affected Models

HP LaserJet 4000
HP LaserJet 4050
HP LaserJet 8100
HP LaserJet 8150
HP LaserJet 9000

Common Causes

EIO card (network card, extra memory, or accessory card) has become loose in its slot
EIO card has failed and is reporting a fault to the printer's formatter board
Incompatible EIO card that was not certified for this specific printer model
Firmware conflict between the printer and the EIO card's embedded firmware
Physical damage to the EIO slot or card from static discharge or improper installation

How to Fix It

Turn off the printer and unplug it from the wall. Locate the EIO slot access panel on the back or side of the printer. EIO slots are usually clearly labeled and have a cover panel secured by one or two thumbscrews.

Always unplug before accessing the EIO slot — EIO cards should never be inserted or removed with the printer powered on.
Remove the EIO card from its slot. Note the card's orientation for reinstallation. Inspect the card and the slot for any obvious physical damage, bent pins, or burn marks. Also check the edge connector on the card for corrosion.

Like memory modules, EIO card contacts can be cleaned gently with a clean pencil eraser to remove oxidation.
Turn the printer on without the EIO card installed (use the blank cover plate to cover the open slot). If the printer starts normally, the EIO card is the problem — either faulty, incompatible, or simply needs to be reseated.

Without a network EIO card, the printer will only be accessible via USB or parallel port — not over a network.
Reseat the EIO card. Turn off the printer, reinsert the card firmly until fully seated in the slot, and secure the thumbscrews. Turn the printer on and check whether error 73 still appears.

EIO cards sometimes work loose over time, especially in environments with vibration (like near other heavy machinery or in active office spaces).
If error 73 persists with the card seated, update the printer's firmware. Firmware updates sometimes include compatibility fixes for EIO cards. Download the latest firmware from HP's support site using your printer model number and follow HP's firmware update instructions.

HP firmware updates for older LaserJets are installed via a parallel port or USB connection to a PC running HP's firmware update utility.

When to Call a Professional

If removing the EIO card and running the printer without it resolves error 73, the card itself is faulty. Replacement HP EIO network cards are available online for $20-$80 depending on the type. If the error persists even with the EIO slot empty, the formatter board may be damaged and a technician should evaluate it.

Frequently Asked Questions

What is an EIO card on an HP LaserJet?

EIO (Enhanced Input/Output) is a proprietary expansion slot found on older HP LaserJet printers. EIO cards can add network connectivity (Ethernet or wireless), extra storage, or other features to a base printer. They were common on HP LaserJet 4000, 5000, 8000, and 9000 series printers from the 1990s and 2000s. Newer HP printers have built-in networking and don't use separate EIO cards.

Can I use any EIO card in any HP LaserJet?

Not always. While most HP EIO cards are designed to be interchangeable, some cards are only certified for specific printer models. Always check HP's compatibility list before installing a third-party or salvaged EIO card. An incompatible card won't physically damage the printer but will cause error 73.

My printer works fine without the EIO card — do I need it?

Only if you need the feature it provides. If the EIO card is a network card and you're not using network printing (printing via USB or parallel port instead), you don't need it. If you do need network printing, replace the faulty card with a known-compatible HP EIO network card. They're available on eBay and Amazon for $15-$50 for older models.