Error Code C1004 is an FNS (Finisher) Communication Error — Engine Detection on Konica Minolta bizhub machines. It fires when the main control board (PRCB or MFPB) detects a communication failure while receiving data from the finisher’s FS Control Board (FSCB) — or when a finisher that should be installed is not being detected at all.

C1004 is the engine-side detection of the finisher communication failure. Its companion code, C1014, is the finisher-side detection of the same fault — the FSCB detecting that it cannot communicate with the main board. When both C1004 and C1014 appear together, the communication link between the machine and the finisher is completely broken. When only C1004 appears, the engine board is detecting the failure but the finisher board may still be partially functional.

C1004 Has Different Component References by Finisher and Model Family

The most important diagnostic fact about C1004 is that the connector paths, fuse references, and relay board designations differ significantly between finisher models and bizhub generations. Before doing anything else, identify your installed finisher and locate the correct references for your combination in the table below.

What Does Error Code C1004 Mean?

The bizhub finisher communicates with the main machine over a serial data bus. When a print job requires finishing (stapling, sorting, punching, or booklet making), the main control board sends job parameters to the FSCB, and the FSCB returns status updates as it processes each set. This communication happens continuously throughout every finishing job.

C1004 fires when this communication breaks down — specifically when the main board is the side detecting the failure. The PRCB or MFPB sends a data request to the FSCB and does not receive a valid response within the expected time window, or receives corrupted data that cannot be processed.

The second trigger — no finisher detected — occurs when the communication link is completely absent at startup. The machine checks for a connected finisher during its boot sequence and, if none is found (even though one should be installed), logs C1004.

The breakdown can occur at several points:

• The inter-unit communication harness between the finisher and the main machine is disconnected or damaged
• The firmware versions on the FSCB and main board are incompatible
• A fuse or ICP in the JSCB or FSCB communication power path has blown
• The FSCB has a hardware fault preventing reliable communication
• The PRCB or MFPB has a fault on its finisher communication receiver circuit

Common Causes of Error Code C1004

• Transient power event — the most common cause of a single C1004. A voltage fluctuation, brief power interruption, or an ESD event during a finishing job corrupts the communication handshake. A full power cycle clears it without any physical repair
• Inter-unit harness connector between finisher and machine body disconnected or loose — the physical cable connecting the finisher to the machine has a loose connector at either end. This is the most common hardware cause of persistent C1004, especially after a finisher has been removed and reattached for service or relocation
• Firmware version mismatch between FSCB and PRCB/MFPB — the second most common cause of persistent C1004. After a firmware update to the main machine, the finisher firmware may no longer be compatible with the new main board firmware version. The protocol handshake fails even though all physical connections are intact. This is especially common after a firmware update is performed without also updating the finisher firmware
• Blown JSCB F1 or F2 fuse (JS-506/JS-508 installations) — when a Job Separator rather than a full finisher is installed, the JSCB’s two inline fuses F1 and F2 are in the power and communication path. A blown fuse completely disables the JSCB, causing C1004 on every startup. This is the most common hardware cause for job separator installations
• Blown FSCB ICP1 or ICP2 (bizhub C3851/C3351) — on these models with an integrated finisher, inline circuit protectors on the FSCB board are in the communication circuit. A blown ICP produces C1004 that does not respond to power cycles or firmware rewrites
• Damaged inter-unit harness — the cable between the finisher and the machine body has been pinched, kinked, or damaged — most commonly at the door hinge or cable routing path where the harness flexes every time the finisher access door is opened
• Failed FSCB (FS Control Board) — the finisher’s main control board has a hardware fault preventing reliable communication
• Failed PRCB or MFPB — the main board’s finisher communication receiver circuit has failed — last resort

Step-by-Step Troubleshooting Procedure

Step 1 — Perform a Full Power Cycle

C1004 is a Rank C error — a single occurrence is expected to clear with a proper power cycle. This is always the mandatory first step and resolves the majority of C1004 occurrences that technicians encounter in the field.

1. Press the main power switch to OFF
2. Disconnect the power cord from the wall outlet — simply pressing the power button is insufficient. The machine’s internal capacitors must fully discharge
3. Wait a full 10 seconds minimum (30 seconds is preferable)
4. Reconnect the power cord
5. Wait 5 more seconds, then press the main power switch to ON
6. Observe the startup sequence — if C1004 does not return and the finisher initialises correctly (you should hear it running its self-test), the event was transient
7. Test all finishing functions — run a staple job, a sort job, and any other finishing functions the customer uses — to confirm stable operation
8. If C1004 returns on the next startup or within the next few print cycles, proceed to Step 2

Step 2 — Verify and Update Firmware on Both Machine and Finisher

Persistent C1004 that returns after a proper power cycle is highly likely to be firmware-related, particularly on machines where a recent firmware update was performed. Before any physical inspection, verify and update firmware — this is the official second step in every service manual for C1004 across all model families.

1. Enter Service Mode and navigate to Machine Information → Firmware Version — record the current engine firmware version
2. Also check the finisher firmware version — on most models this is accessible from Service Mode → Machine Information → Option Firmware or equivalent
3. Contact your Konica Minolta distributor or check the service portal for the latest firmware package for your specific machine model
4. The firmware package typically includes both the engine firmware AND the finisher firmware — download the complete package
5. Update the engine firmware first following the standard procedure for your model
6. Then update the finisher firmware if a separate finisher firmware update is available
7. After all updates, perform a full power cycle (disconnect cord, wait 10 seconds) before testing
8. If C1004 persists after confirmed successful firmware updates to the latest version, the issue is hardware — proceed to Step 3

Step 3 — Inspect and Reseat the Inter-Unit Harness Connector

The physical cable connecting the finisher to the machine body is the most common hardware cause of persistent C1004. This connector is disconnected every time the finisher is removed for service or relocation — and even a slight misalignment during reattachment can cause C1004.

1. Power OFF and unplug the machine
2. Locate the inter-unit connector — this is the cable that runs between the finisher and the main machine body, typically exiting through a slot or opening at the connection point where the finisher attaches to the machine
3. Disconnect the connector completely at both ends — at the finisher side and at the machine body side
4. Inspect every pin in both connectors carefully under good lighting:
   • Look for pins that have backed out of the connector housing
   • Check for bent pins
   • Look for green or white oxidation on the metal contact surfaces
   • Check for damage to the harness wires at bend points near the connector
   • Check for any mechanical damage to the connector housing from repeated connection/disconnection
5. Clean any oxidized pins with electrical contact cleaner on a cotton swab — allow to dry completely before reconnecting
6. Reconnect both ends of the inter-unit harness firmly — push until you hear or feel the connector click into the locked position
7. On floor-standing finishers, also check that the finisher is correctly docked to the machine — an improperly docked finisher will have the connector engaged but with mechanical misalignment that causes intermittent contact
8. Power on and test all finishing functions

Step 4 — Inspect the Harness Routing for Physical Damage

If the connector is confirmed seated but C1004 persists, inspect the full length of the inter-unit harness for damage that is not visible at the connector ends. This is particularly important on finishers with access doors that open and close frequently, as the harness flexes at the door hinge with every opening.

1. Trace the complete harness from the FSCB inside the finisher to the main board connector on the machine body
2. Check every point where the harness passes through a panel, routing clip, or grommet — these points are where pinching and abrasion damage occur
3. Open and close the finisher access door slowly while watching the harness — confirm there is no point where the harness is being pinched, pulled taut, or rubbed against a sharp edge during door movement
4. On floor-standing finishers with relay units, check the harness where it crosses through the relay unit connection point as well
5. If any section of the harness is visibly damaged — cracked insulation, exposed conductors, or a clearly kinked section — replace the harness. Do not attempt to splice or repair communication harnesses as impedance mismatches from repairs cause intermittent errors

Step 5 — Check JSCB F1 and F2 Fuse Continuity (Job Separator Installations)

When a Job Separator JS-506 or JS-508 is installed instead of a full finisher, this step is the most important hardware check for C1004. The JSCB has two inline fuses — F1 and F2 — that blown fuses on these cause C1004 on every startup regardless of harness condition or firmware version.

1. Access the JSCB (Job Separator Control Board) inside the job separator unit — typically accessible by removing the unit’s rear cover panel
2. Locate fuses F1 and F2 on the JSCB board — refer to your service manual for exact component locations
3. Test both fuses with a multimeter in continuity mode:
   • Continuity present → fuse is intact
   • No continuity (open circuit) → fuse has blown
4. If either fuse has blown, investigate what caused the overcurrent before replacing:
   • A short circuit in the inter-unit harness can blow F1/F2 — inspect the harness before fitting a new JSCB
   • A power surge can blow both fuses simultaneously — check the machine’s power supply for unusual voltage events
5. A blown fuse requires JSCB board-level repair or replacement of the complete JSCB
6. After replacing the JSCB, retest harness continuity before powering on to prevent immediate re-blowing

Step 6 — Check FSCB ICP1 and ICP2 Continuity (bizhub C3851 / C3351)

On the bizhub C3851 and C3351 with the integrated FS-533 finisher, the FSCB has two inline circuit protectors — ICP1 and ICP2 — in the communication circuit. These are not present on other model families, making them a C3851/C3351-specific check.

1. Access the FSCB inside the integrated finisher section of the machine
2. Locate ICP1 and ICP2 on the FSCB — refer to the C3851/C3351 service manual for the exact locations
3. Test both ICPs for continuity — no continuity on either confirms it has blown
4. A blown ICP requires FSCB board-level repair or full FSCB replacement
5. Also verify the connector path: FSCB CN1 → relay CN600 → MFPB CN16E and FSCB CN4 → relay CN610 → DCPU CN11 — reseat both paths before and after ICP testing

Step 7 — Replace the FSCB (FS Control Board)

If firmware is confirmed current, all connectors are confirmed seated, the harness is confirmed undamaged, and JSCB fuses / FSCB ICPs are confirmed intact — the FSCB itself has a hardware fault preventing reliable communication.

1. Order the correct FSCB for your specific finisher model — FSCB part numbers differ between FS-533, FS-534, FS-535, FS-536, FS-537, FS-539, and FS-540, and between finisher generations
2. Power OFF and unplug the machine before accessing the FSCB
3. Access the FSCB — on most finishers it is accessible by removing the finisher’s rear cover. On floor-standing finishers it may require removing additional panels
4. Disconnect all harnesses from the FSCB, noting their positions before removal
5. Remove the FSCB mounting screws and extract the board
6. Install the replacement FSCB, reconnect all harnesses in the correct positions, and confirm all connectors are fully seated
7. Power on and immediately update the FSCB firmware to the current version — replacement boards often ship with older firmware that may not be compatible with the current engine firmware
8. Test all finishing functions — staple, sort, punch (if installed), booklet (if installed) — to confirm full finisher operation is restored

Step 8 — Replace the PRCB or MFPB

Main board replacement is the final resort for C1004 and should only be performed after a confirmed-good FSCB, confirmed harness, confirmed firmware compatibility, and confirmed fuse/ICP continuity still produce persistent C1004.

• PRCB — bizhub C224/C284/C364 family, C454/C554, C220/C280/C360, C552/C652/C452, 283/363/423, 654/754
• MFPB — bizhub C227/C287/C367, C258/C308/C368, C458/C558/C658, 458e/558e/658e, C450i/C550i/C650i, C3851/C3351

Quick Reference — Troubleshooting by Symptom

Understanding the C100x Finisher Communication Error Family

C1004 belongs to a group of finisher-related communication errors. Understanding the full family helps diagnose related codes that appear alongside C1004, and prevents wasted time checking the wrong board:

• C1003 — FNS communication error (engine detection, on some older model families — same as C1004 but on different finisher model pairings)
• C1004 — FNS communication error — Engine Detection (PRCB/MFPB detecting the failure)
• C1014 — FNS communication error — Finisher Detection (FSCB detecting the failure on its side)
• C1081 — SD (Saddle) communication error — finisher detection (when FS-539SD or FS-540SD is installed)
• C1082 — SD communication error — saddle unit detection
• C1007 — PI (Punch) communication error — finisher detection (when PI-507 is installed with FS-540)
• C1013 — PK (Punch Kit) communication error (when PK-526 is installed)

C1004 + C1014 appearing together: Both the engine and finisher are detecting the same communication failure simultaneously. This means the communication link is completely broken — most likely a fully disconnected inter-unit harness or a failed FSCB. Fix the harness first.

C1004 without C1014: The engine is detecting the failure but the finisher board may still be partially active. Firmware mismatch is a more likely cause in this scenario than a complete physical disconnection — update firmware before replacing hardware.

C1004 + C1081 together: Both the main finisher and the saddle unit are losing communication simultaneously. This points to a shared power or communication fault in the finisher — the FSCB is the common point between the main finisher and the saddle sub-unit. Replace FSCB.

Why Firmware Is So Critical for C1004

C1004 is one of the most firmware-sensitive errors on the bizhub platform — second only to C0002 in this regard. The reason is that the communication protocol between the engine board and the FSCB is defined in firmware on both sides. When the protocol versions do not match:

• The handshake at startup fails even when all physical connections are perfect
• The engine board sends a request in the new protocol format — the FSCB does not understand it — and C1004 fires
• Replacing the FSCB without updating firmware produces the same error on the new board

This is why the official service manual remedy lists firmware rewriting as Step 2 — before any connector checks and before any board replacement. In field practice, a firmware update resolves a significant portion of persistent C1004 cases faster and at zero parts cost compared to any hardware repair.

The most common scenario: a firmware update is performed on the main machine. The update process does not automatically update the finisher firmware. The engine now runs protocol version N+1 but the FSCB still runs protocol version N. C1004 fires on every startup. The fix: update the finisher firmware to match.

Preventing C1004 From Recurring

• Always update finisher firmware when updating engine firmware — make this a mandatory part of your firmware update procedure. C1004 from a firmware mismatch is entirely preventable if both boards are updated together
• Reseat the inter-unit harness connector at every PM visit — this connector is the most common hardware cause of C1004 and is disconnected every time the finisher is removed for service. A 30-second reseat and visual inspection during PM prevents the most common hardware-related C1004 calls
• Inspect the harness routing at the finisher access door hinge during PM — the communication cable flexes every time the finisher access door is opened during jam clearance. On high-volume machines with frequent paper jams through the finisher, this harness section is subject to fatigue damage. A quick check during PM catches developing damage before it causes C1004
• Ensure stable, clean power supply — machines on circuits with voltage fluctuations, shared heavy equipment, or long extension leads experience more transient C1004 events from power-related communication disruptions
• After FSCB replacement, always update firmware immediately — a replacement board with older firmware will produce C1004 on the first startup. Build firmware update into your FSCB replacement procedure as a mandatory final step
• Test all finishing functions at the end of every PM visit — run a test staple job and a sort job as part of your standard PM sign-off. C1004 from a displaced harness connector during PM is entirely preventable with this 2-minute check

Professional Technician Summary

Error Code C1004 on Konica Minolta bizhub machines is a finisher communication error detected on the engine side, and its resolution path closely mirrors C0002 — it is fundamentally a communication failure, not a mechanical one, and firmware plays an unusually large role in diagnosis.

A single C1004 occurrence is almost always transient and clears with a proper power cycle. Do not replace hardware on a machine that has produced C1004 once.

Persistent C1004 should be addressed with a firmware update before any hardware replacement — particularly after any recent engine firmware update that may have left the finisher firmware on an older version. This single step resolves a large proportion of persistent C1004 cases at zero parts cost.

When firmware is confirmed current and C1004 persists, the diagnostic priority is: inter-unit harness connector → harness physical inspection → JSCB F1/F2 fuses (job separator) or FSCB ICP1/ICP2 (C3851/C3351) → FSCB replacement → PRCB/MFPB replacement as absolute last resort.

Always update the FSCB firmware immediately after board replacement — a replacement FSCB with older firmware will produce C1004 on the first startup, and this mistake is frequently misdiagnosed as a defective replacement board.