Common Causes of E4 Fatal Error on DTF-Modified Epson L1800 Printers

Question

A customer shared that after performing several diagnostic steps on their DTF-modified Epson L1800, they noticed several concerning symptoms:

• All five diagnostic tests in the Adjustment Program now return E4 Fatal Error codes.

• The LED on the white-ink circulation/fan board did not light up, although the PF sensor LED did.

• The power supply unit labeled MODEL S-75-24 / EPS-155E did not show its green LED.

• A red LED on the board they refer to as the "RT Main Board" also did not light up.
  They also confirmed that they tested cables, checked FFC connections, inspected every board they could access, and performed multimeter checks that showed voltage spikes where expected. They ask what these symptoms collectively mean, whether board replacement is likely, and whether there are videos showing easier access to components like the power supply. They are becoming more comfortable doing this type of repair and want guidance before purchasing new parts.

Answer

Thank you also for the kind words - they mean a great deal. And your diagnostic work has been exceptionally thorough. Your documentation, LED observations, careful multimeter testing, and attention to detail all read like the work of someone who has been repairing these machines for years. That level of care gives us a very clear path toward interpreting what your printer is doing.

Below is a detailed breakdown of what each symptom tells us, what the E4 Fatal Error code indicates, and which component is the most likely culprit.

Understanding the Symptoms and Error Codes

1. E4 Fatal Error on All Five Tests

The E4 Fatal Error on Epson L1800-based systems usually indicates:

• A power-delivery failure

• A communication failure between boards

• Or a critical mechanical sensor fault

However, when all five functions return E4 at the same time, that is a strong indicator of a shared upstream failure - meaning one component that everything depends on has stopped working.

This already points toward a power-supply-related problem.

2. White-Ink Circulation Board LED Not Lighting

Even if the circulation motor or stirrer isn't operating, the circulation control board should still show a standby LED when power is present.

No LED suggests:

• No incoming voltage on its rail

• A broken trace or failed regulator

• Or lack of power from the relay/power-distribution board

This symptom matches perfectly with a partial loss of the 24V rail.

3. PF Sensor LED Illuminates Normally

This is actually an important clue.

The PF sensor runs on the 5V logic rail, which means:

• The printer is still receiving low-voltage logic power

• The 5V supply from the mainboard is not dead

• The failure is not a complete system shutdown

This helps narrow the issue to just the higher-voltage line.

4. Power Supply (S-75-24 / EPS-155E) Green LED Is Off

This is the most significant finding.

Your power supply provides the 24V high-voltage rail, which powers:

• The pump motor

• Carriage motor

• Fans

• The white-ink circulation system

• Certain relays

• Sub-controllers and switching boards

If the green LED is off, then the 24V rail is either:

• Dead,

• Intermittent, or

• Collapsed under load

When 24V disappears, the printer appears to "boot," but any motor-dependent diagnostic test from the Adjustment Program fails instantly with E4 Fatal Error - exactly what you are experiencing.

5. "RT Main Board" Red LED Not Lighting

This board is typically a relay/power-distribution board. It depends entirely on the 24V rail from the main power supply. If the supply is down:

• This board cannot activate

• Its LEDs remain off

• Downstream components never receive power

This confirms the failure is upstream, not in these smaller boards.

Most Likely Root Cause: Failed 24V Power Supply

All symptoms point to a single, highly probable cause:

👉 The 24V power supply (EPS-155E / S-75-24) has failed.

This is extremely common in DTF-converted printers because:

• The added circulation systems increase load

• The units often run for long cycles

• Heat buildup stresses internal regulators

• Sudden 24V failure causes multiboard "cascading" symptoms

When this power supply collapses, many printers show almost identical behavior to what you observed.

Could Other Boards Be Damaged?

Possibly - but not necessarily.

Based on experience:

• Most boards come back to life once a stable 24V supply is restored.

• If any boards were affected, they typically show failure after a good power supply is installed.

So the proper order is:

1. Replace the 24V PSU

2. Power on and recheck LEDs and behavior

3. Only investigate additional boards if symptoms remain

This avoids unnecessary part replacement.

Tool Access & Removing Components

Your approach and the tools you mentioned are perfect. Many technicians use the same method.

Helpful tools include:

• A stubby Phillips screwdriver

• A flexible extension driver

• Needle-nose pliers

• A magnetic pickup tool

• Your mini ratchet is excellent for lower-frame screws

On most L1800 conversions, the "silver box" power supply is mounted low and partially behind the feed assembly. Typically:

• The lower feed assembly must be loosened or removed for access

• There is no universal shortcut because conversions vary

• Many third-party DTF shops mount components differently

For ease of finding videos, the fastest method is to use the search function on our YouTube channel:
Search "L1800 Power Supply," "L1800 disassembly," or "DTF power supply replacement."
You can do this directly on our channel using the magnifying-glass icon on the top right:
BCH Technologies on YouTube (https://youtube.com/@bchtechnologies)

This is always more accurate than my memory, since we've produced so many videos over the years.

Recommended Next Steps

1. Replace the 24V EPS-155E / S-75-24 power supply with a known-good unit.

2. Reconnect and verify LEDs on:

   • Circulation board

   • Relay/power distribution board

   • New power supply

3. Re-run the Adjustment Program.

4. If E4 remains, begin testing secondary boards.

Given the symptoms, chances are extremely high that restoring the 24V rail will revive most, if not all, subsystems.

Addressing printer issues can sometimes be complicated because of the hands-on nature of these repairs. That's why we aren't able to provide remote troubleshooting or hands-on repair support. We do offer in-person diagnosis and repair through our local service facility at the BCH Diagnostics Center - you can find details here: Printer Repair Service (https://bchtechnologies.com/printer-repair-service). Due to demand, visits are handled on a first-come, first-served basis, and it may take a few weeks to receive a slot. Our services cover full-printer or partial repairs, but we understand our rates may not be the most budget-friendly. For this reason, we strongly recommend continuing self-help through online research.
You can start by visiting our YouTube channel's homepage: BCH Technologies on YouTube (https://youtube.com/@bchtechnologies). Use the search icon next to "About" on the right side of the tab bar to find videos on specific topics. With hundreds of videos over the past decade, using YouTube's built-in search is the fastest way to locate relevant guides - and YouTube may also suggest helpful videos from other creators.

Thank you again for your diligence, patience, and dedication. Your careful approach and detailed observations make troubleshooting far more effective. Whenever you're ready for the next step - or if you need to check in about ink tanks or accessories once this issue is fixed - feel free to reach out anytime.