HP DesignJet T520 Repair: Error 86:01, 87:10 & Power Issues

PRGPRG Technical Team·Updated July 2026Error-Code Guides·10 min read

The HP DesignJet T520 is a reliable wide-format plotter — until it isn't. Our technicians have worked through 30+ T520 service calls across the PRG network, and the same three failure modes keep showing up: error 87:10 from a loose starwheel spring, a no-power condition with a flashing red light, and a main board failure that kills printing entirely. This guide covers all three, including the encoder strip cleaning procedure that shows up as a first-step diagnostic across nearly every wide-format plotter call we run.

The short version

Error 87:10 almost always traces to a loose spring in the starwheel assembly — reinstall it, then clean and lube the scan rail and encoder strip.
A flashing red power light with no boot means power supply failure; rule out the wall outlet and cord first, then replace the PSU.
A main board failure shows as a printer that receives jobs but will not print — no partial output, no movement, no response.
Encoder strip contamination is a root cause and a complicating factor across all three modes; cleaning it is never wasted effort.

Breakdown of the three dominant T520 failure modes seen across PRG service calls.

Error 87:10 — the starwheel spring and why it matters

Error 87:10 is a carriage/scan-axis fault. On the T520, it consistently points to a single mechanical cause: the spring that tensions the starwheel assembly has come loose. When that spring loses its seat, the starwheel can't maintain proper contact with the media, the encoder strip reads inconsistent carriage position, and the printer throws 87:10 to stop itself from crashing.

The fix has three steps. Skip any one of them and the error comes back.

Step 1 — Locate and reinstall the spring

Open the top cover and slide the carriage to the center of the rail by hand (power off first). The starwheel assembly sits along the front of the carriage path. Inspect the small tension spring at each end of the starwheel rod. One will be unhooked or fully dislodged. Use fine-tip needle-nose pliers to reseat it on its anchor post. You should feel it click into tension — if it springs back off under light pressure, the post is worn and the assembly needs replacement.

Step 2 — Clean and lubricate the scan rail

With the carriage away from the home position, wipe the full length of the scan rail with a lint-free cloth lightly dampened with isopropyl alcohol (90%+). The rail accumulates ink mist, paper dust, and dried lubricant over time. After cleaning, apply a thin bead of PTFE-based or HP-recommended scan-axis grease to the rail. Work the carriage back and forth by hand to distribute it evenly. Avoid silicone sprays — they attract debris.

Step 3 — Clean the encoder strip

The encoder strip is the clear or lightly tinted plastic strip that runs the full width of the carriage path, just behind the rail. The carriage's optical sensor reads position marks printed on this strip. Ink mist, dust, and handling smudges degrade the signal — and a degraded signal produces carriage errors like 87:10 even when the mechanical cause has already been fixed.

To clean it: fold a lint-free cloth around the strip (never touch it bare-handed) and wipe from left to right in one direction only. Use IPA on the cloth — not directly on the strip. Check under a flashlight for remaining smears. A clean strip should be uniformly clear with no visible smudging. This procedure applies across nearly every wide-format plotter we service, not just the T520 — it's the first thing our field technicians check on any carriage-axis error.

After completing all three steps, power the printer on and run a carriage calibration from the front panel menu (Setup → Printer Maintenance → Calibrate). If 87:10 returns, the encoder strip sensor on the carriage board may need replacement.

No-power condition — flashing red light diagnosis

The second failure mode is a T520 that won't boot. The power button flashes red — sometimes continuously, sometimes in a timed pattern — and the printer never initializes. No display, no carriage movement, no fan spin-up.

Before condemning the power supply, eliminate the easy stuff:

Test the wall outlet with a known-good device. Surge protectors can fail silently; plug directly into the wall.
Swap the power cord. Wide-format plotters use a standard IEC C13 cord — test with a verified spare.
Check the input voltage. The T520 auto-ranges (100–240V), but a severely degraded outlet can supply just enough voltage to attempt a start, then drop.

If the outlet and cord check out, the power supply unit (PSU) is the confirmed failure. The T520's PSU is an internal module located at the rear of the unit behind the access panel. Replacement requires disconnecting the main board harness and the print-engine power cable — a straightforward swap but one that warrants care around the ribbon connectors. Forcing a ribbon connector is the most common secondary damage we see on PSU replacements.

A flashing red light that stops after a firmware reset (hold power + cancel during boot) suggests the PSU is marginal but functional — in that case, monitor it closely and plan for replacement within a few months. A light that never stops flashing regardless of input is a hard PSU failure.

Main board failure — print jobs disappear with no output

The third failure mode is the most disruptive: the T520 appears fully functional, accepts print jobs from the host, but nothing ever prints. The front panel shows no job in queue, the carriage never moves, and there are no error codes.

This symptom pattern points to main board failure. The board receives the job at the network or USB interface layer but cannot pass it to the print engine. Distinguishing this from a driver or firmware issue is the first diagnostic step.

Rule out software causes first

Print the internal demo page from the front panel. If it prints, the board and engine are functional and the problem is upstream (driver, spool, network).
Flash the latest firmware via USB, not the network — a corrupted firmware image can produce this exact symptom.
Factory reset the printer (Setup → Restore Factory Settings) and retest with a direct USB connection from a single PC.

If the demo page does not print and firmware flash makes no difference, the main board is the diagnosis. On the T520, the main board and the formatter board are integrated into a single assembly. Replacement is the only repair path — there is no field-level component repair for this board.

A note on encoder strip contamination and board failures

In several of our T520 main board calls, encoder strip contamination was a contributing factor — not the primary cause, but a stressor. A heavily contaminated strip forces the carriage servo to work harder and longer, generating heat and electrical noise that wears out board components faster. Keeping the encoder strip clean is cheap maintenance insurance whether or not you're currently seeing error codes.

Error 86:01 — scan-axis shutdown explained

Error 86:01 is the T520's scan-axis motor shutdown code. It fires when the carriage motor draws excessive current — which happens when the carriage encounters mechanical resistance it can't overcome.

The most common causes, in order of likelihood:

Dry or contaminated scan rail. This is the first thing to address. Clean and re-lubricate as described in the 87:10 section above.
Dirty encoder strip. A degraded encoder signal causes the motor controller to over-drive the motor trying to confirm position. Clean the strip before drawing any other conclusions.
Foreign object in the carriage path. Paper fragments, small clips, and torn media edge pieces are common. Inspect the full carriage path with a flashlight.
Carriage belt wear or tension. A frayed or slack belt adds drag. Inspect the belt for cracking or glazing and confirm the tensioner is seated.
Carriage motor failure. If the above steps don't resolve the error, the scan-axis motor itself may be drawing excess current under load.

Errors 86:01 and 87:10 often appear together or in sequence on the same unit. That's not a coincidence — they share the same mechanical subsystem, and an unresolved scan-rail or encoder issue can trigger both codes at different points in the carriage movement cycle.

When it's time to call a technician

The starwheel spring reinstall and encoder strip cleaning are tasks most in-house IT staff can handle with basic tools and care. The power supply replacement is manageable if you're comfortable with internal hardware. The main board swap, however, involves a formatter/main board assembly that costs several hundred dollars — and mishandling the ribbon connectors can turn a board replacement into a board replacement plus a carriage assembly. At that point the repair cost approaches the printer's replacement value, and you want a technician making that call, not discovering it mid-disassembly.

Call for on-site printer repair if any of the following apply:

Error 86:01 or 87:10 returns within a day of cleaning and lubricating the scan rail — the carriage belt or motor may be failing.
The red power light flashes and does not respond to a firmware reset — confirmed PSU failure requiring parts and internal access.
The printer accepts jobs but will not print after firmware reflash and factory reset — this is a main board diagnosis that needs professional confirmation before you order a $300+ part.
You find physical damage to the encoder strip (creases, tears, or ink saturation) — the strip must be replaced, not cleaned, and alignment matters.
The carriage grinds or skips during movement after lubrication — the carriage assembly or belt may have sustained mechanical damage.

Wide-format plotters like the T520 are straightforward, but a misread diagnosis or dropped connector can escalate cost fast. When the error pattern doesn't match a clean single cause, having a technician on-site to run the full diagnostic is the faster and cheaper path.

Root causes behind T520 scan-axis faults across HP wide-format service history — causes can co-occur.

Frequently asked questions

What does error 87:10 mean on the HP DesignJet T520?

Error 87:10 is a carriage scan-axis fault triggered by the printer losing confidence in carriage position. On the T520, the most consistent cause is a loose or dislodged spring in the starwheel assembly. Reinstalling the spring, cleaning the scan rail, and cleaning the encoder strip resolves the error in most cases. If it returns, the encoder strip sensor on the carriage board may need replacement.

What is error 86:01 on the HP DesignJet T520?

Error 86:01 means the scan-axis motor shut down due to excess current draw. The motor is working too hard against mechanical resistance. Start with a scan-rail cleaning and lubrication, then clean the encoder strip. If those steps don't resolve it, inspect the carriage belt for wear and check for foreign objects in the carriage path. A motor that continues to over-draw after those steps is failing and needs replacement.

Why is the power light on my HP DesignJet T520 flashing red?

A flashing red power light on the T520 with no boot sequence almost always indicates power supply failure. First, verify the wall outlet is live and swap the power cord with a known-good IEC C13 cable. If the light still flashes after those checks, the internal PSU needs to be replaced. A flashing light that clears after holding power and cancel during boot suggests a marginal PSU — it may function temporarily but will fail completely in the near term.

My T520 shows jobs in the queue but won't print. What's wrong?

If the T520 accepts jobs but produces no output — no carriage movement, no error codes, nothing — start by printing the internal demo page from the front panel. If the demo page doesn't print either, attempt a firmware flash via USB and a factory reset. If neither resolves it, the main board (formatter/main board assembly) has failed and needs to be replaced. This is a parts-intensive repair worth having a technician confirm before ordering components.

How do I clean the encoder strip on an HP DesignJet T520?

Power the printer off. Slide the carriage to the center of the rail by hand. Locate the clear or lightly tinted plastic strip running the full width of the carriage path behind the rail. Fold a lint-free cloth around the strip and wipe from left to right in one direction — do not use a back-and-forth motion, as this can redistribute contamination. Apply isopropyl alcohol (90%+) to the cloth, not directly to the strip. Inspect under a flashlight for remaining smears. Never touch the strip with bare hands; skin oils degrade the optical markings.

Can errors 86:01 and 87:10 appear on the same printer?

Yes, and it's common. Both codes involve the scan-axis subsystem, so an unresolved mechanical issue — a dirty rail, a contaminated encoder strip, a worn belt — can trigger both codes at different points in the carriage cycle. Resolve the underlying cause rather than chasing each code individually. Start with the scan-rail cleaning and encoder strip cleaning described above; in most cases that addresses both errors at once.