HP Printer Error 91 — What It Means & How to Fix It

What Does This Error Mean?

HP Error 91 indicates a fuser temperature error — the fuser is overheating or failing to reach the correct temperature. This is closely related to Error 90, but Error 91 specifically points to a thermal runaway or temperature regulation failure. The printer shuts down immediately to prevent a fire hazard. The fuser assembly and its thermistors are the primary suspects.

Affected Models

HP LaserJet 4200/4250/4300 series
HP LaserJet Enterprise 600 M600 series
HP LaserJet 9000/9040/9050 series
HP Color LaserJet Enterprise CP4525
HP LaserJet P4014/P4015/P4515 series

Common Causes

Fuser thermistor has failed and is sending an incorrect temperature reading — making the fuser heat too much or too little
Fuser heating element is failing and cannot maintain a stable temperature
Fuser assembly has reached end of life and temperature regulation has become erratic
AC power supply voltage is incorrect or unstable, affecting fuser heating consistency
DC controller board no longer properly regulates the fuser's heating circuit

How to Fix It

Immediately power off the printer. Let it cool for at least 30 to 45 minutes before touching any internal components. The fuser retains heat long after the printer powers down.

Error 91 indicates a thermal regulation problem. Continuing to operate the printer creates a potential fire risk. Always power off immediately.
Check the power supply. Plug the printer directly into a wall outlet — not a power strip or surge protector. Voltage irregularities from shared circuits can affect fuser temperature control.

Large laser printers draw significant power during fuser warm-up. A power strip or extension cord can cause voltage drops that disrupt fuser temperature regulation.
Remove the fuser assembly after the printer has cooled. Check the fuser's electrical connector for burn marks, bent pins, or melted plastic. These indicate a serious fuser fault.

Any sign of burning or melting means the fuser has failed catastrophically and must be replaced — do not reinstall it.
Install a new compatible fuser assembly for your specific printer model. Ensure it is correctly matched to your printer's voltage requirements.

Using a 110V fuser on a 220V supply causes immediate overheating and can trigger Error 91 on the very first use.
If Error 91 returns with a brand-new fuser, the DC controller board is likely faulty. The DC controller regulates the AC power to the fuser's heating element. A failed controller cannot maintain stable fuser temperature.

DC controller replacement is an advanced repair. Contact HP support or an authorized service center.

When to Call a Professional

Error 91 is a safety-critical error requiring immediate attention. Do not attempt to continue printing and do not bypass the error. Fuser replacement resolves most Error 91 cases. If a new fuser does not resolve it, the DC controller needs professional service — expect $200 to $500.

Frequently Asked Questions

What is thermal runaway in a laser printer?

Thermal runaway happens when the fuser's temperature control circuit fails and the heating element keeps getting hotter without stopping. Normally the thermistor measures temperature and cuts off heating at the set point. If the thermistor or the control circuit fails, the fuser can overheat dangerously. The printer's safety circuit detects this and logs Error 91 to shut the printer down before any damage occurs.

Is Error 91 dangerous?

It can be if the printer continues to operate with a faulty fuser. The printer's safety system is designed to shut down before dangerous temperatures are reached. However, if the safety circuit itself fails, overheating is a real risk. Always take Error 91 seriously and do not attempt to override or ignore it.

How do I check the fuser thermistor on an HP printer?

A thermistor is a resistor that changes its resistance with temperature. With the printer cool and unplugged, remove the fuser and test resistance across the thermistor terminals with a multimeter. At room temperature, most fuser thermistors read between 50,000 and 300,000 ohms. An open or shorted reading means the thermistor has failed and needs replacement.