Error Code C2564 is a Black TCR (Toner Concentration Ratio) sensor failure that appears across a wide range of Konica Minolta bizhub colour and mono machines. It signals that the machine’s automatic toner concentration adjustment system has detected an alarm condition from the black TCR sensor — either the sensor cannot read a valid toner density value, or the sensor connector has been flagged as disconnected or open-circuit more times than the allowed threshold.

This error is tightly related to its sibling codes: C2561 (Cyan TCR sensor failure), C2562 (Magenta TCR sensor failure), and C2563 (Yellow TCR sensor failure). All four share the same detection logic and the same component chain — only the colour channel and connector pin differ. Understanding C2564 means you understand all four.

This guide covers every affected model and board variant, explains exactly what the TCR sensor does and why it fails, and provides a complete technician-level troubleshooting procedure from connector inspection through board replacement.

All Affected Models and Board Variants

C2564 is documented across multiple bizhub generations. The relay board designation and connector references differ between model families — use the table below to identify your machine’s specific component path before starting any diagnosis.

What Is the TCR Sensor and What Does It Do?

The TCR (Toner Concentration Ratio) sensor is a small magnetic or optical sensor embedded inside the developing unit. Its job is to continuously monitor the ratio of toner to carrier beads inside the developer mix and report this value to the printer control board.

The control board uses this data to regulate toner replenishment — when the TCR reading drops below a threshold, it commands the toner supply motor to feed more toner from the cartridge into the developing unit. When the reading is within range, no additional toner is added. This automatic adjustment process is called ATDC (Automatic Toner Density Control).

C2564 fires when this adjustment system breaks down for the black channel. Specifically, the machine detects one of two failure conditions:

• Alarm signal threshold exceeded — the TCR sensor has returned an out-of-range or invalid reading more times than the preset consecutive alarm limit. This typically means the sensor inside the developing unit has degraded, the developer mix is severely depleted, or the toner supply to the developing unit has failed
• Connector open-circuit detected — the machine’s self-diagnostic detects that the TCR sensor connector is not making proper electrical contact. The sensor signal line reads as open-circuit, which the machine interprets as a disconnected or failed sensor

Common Causes of Error Code C2564

Before starting the step-by-step procedure, understanding the root cause landscape helps you move faster through diagnosis:

• Developing unit not fully seated — the most common cause of C2564 in the field. The developing unit has a multi-pin connector that must fully engage with the front relay board connector. If the unit is even slightly unseated, the TCR sensor connector reads as open-circuit and C2564 fires immediately
• Oxidized or damaged connector pins — the connector between the developing unit and the FRB/FREYB is subject to oxidation in humid environments and physical damage if the developing unit has been removed and reinstalled many times
• Severely depleted developer mix — if the developer material inside the developing unit has degraded past its useful life, the carrier-to-toner ratio becomes uncontrollable and the TCR sensor consistently reads out-of-range values
• Toner supply blockage — if toner is not reaching the developing unit from the cartridge (blocked toner path, empty cartridge, failed toner supply motor), the developer mix becomes toner-starved and the TCR reading drops below the alarm threshold
• Failed TCR sensor inside the developing unit — the sensor itself has failed electronically, returning a fixed or invalid value regardless of the actual developer state
• Faulty Front Relay Board (FRB / FREYB / PHRYB) — the relay board that passes the TCR sensor signal to the control board has a failed signal path
• Faulty PRCB or MFPB — in rare cases the main control board cannot correctly read or process the incoming TCR signal

Step-by-Step Troubleshooting Procedure

Step 1 — Reinstall the Black Developing Unit

Always start here. A developing unit that is not fully seated is responsible for the majority of C2564 cases in the field, and this step takes under two minutes.

1. Power the machine fully OFF and unplug the power cord
2. Open the front cover of the machine and locate the black (K) developing unit — on colour machines it is typically the rightmost unit when viewed from the front
3. Grip the developing unit handle and pull it straight out until it stops — do not force it past the stop point
4. Before reinserting, inspect the multi-pin connector on the back of the developing unit: check for bent pins, toner contamination on the contacts, or visible physical damage
5. Inspect the mating connector on the front relay board (FRB CN13 for most models, FREYB CN4 for C220/C280/C360 series) for the same issues
6. If toner contamination is present on either connector, clean gently with a dry cotton swab — do not use liquids on these contacts
7. Reinsert the developing unit firmly and evenly — push it in until the front edge is flush with the machine frame and the locking latch (if present) clicks into place
8. Close the front cover, reconnect the power cord, and power on
9. If C2564 clears, run a test print to confirm normal operation — also run a gradation adjustment / colour calibration to ensure the TCR system is reading correctly after reinsertion

Step 2 — Check and Reseat the Harness Connectors

If reinstalling the developing unit does not resolve C2564, systematically check every connector in the TCR signal path for your model variant.

For bizhub C224 / C284 / C364 / C224e / C284e / C364e / C454e / C554e / C454 / C554:

1. Check and reseat: Developing unit → FRB CN13 (black TCR signal)
2. Check and reseat: FRB CN1 → PRCB CN7
3. Check and reseat: FRB CN3 → PRCB CN9

For bizhub C258 / C308 / C368 / C458 / C558 / C658:

1. Check and reseat: Developing unit → FRB CN13 (black TCR signal)
2. Check and reseat: FRB CN1 → MFPB CN4E
3. Check and reseat: FRB CN3 → MFPB CN11E

For bizhub C220 / C280 / C360:

1. Check and reseat: Developing unit → FREYB CN4 (black channel), CN6, CN8, CN10
2. Check and reseat: FREYB CN1, CN2 → PRCB CN4, CN5

For bizhub C654 / C754:

1. Check and reseat: Developing unit /K → PRCB CN2
2. Check and reseat: Imaging unit /C → PRCB CN3; Imaging unit /M, /Y → PRCB CN4, CN5

For bizhub 363 / 423 (mono):

1. Check and reseat: Developing unit contacts (direct contact, no harness on some models)
2. Check and reseat: TCRSB CN29 → relay CN28 → PRCB CN24
3. Also check: M3 → PRCB CN8 and M4 → relay CN13 → PRCB CN8 (developing motor connectors that affect developer mix agitation and indirectly affect TCR readings)

For all models: after reseating connectors, inspect every pin carefully under good lighting. Look for pins that have backed out of the connector housing, pins that are bent sideways, green or white oxidation on contact surfaces, and hairline cracks in connector housings. Clean oxidized contacts with electrical contact cleaner on a cotton swab and allow to dry fully before reconnecting.

Step 3 — Check the Toner Supply and Developer Mix Condition

A TCR sensor that is working correctly but consistently reading out of range is not a sensor failure — it is a developer condition problem. Before replacing any electronic components, confirm that the developing unit is receiving toner and that the developer mix is in usable condition.

1. Check the black toner cartridge — confirm it is not empty and that it is genuine Konica Minolta toner. Compatible toner cartridges with incorrect particle sizes are a common cause of toner supply irregularities that trigger C2564
2. Inspect the toner supply path from the cartridge to the developing unit for blockages — toner bridges, clumped toner, or a failed toner supply auger will starve the developing unit
3. Check the toner supply motor (M3/M4 on mono models) for correct operation in service mode — a failed toner supply motor means no toner reaches the developer regardless of cartridge condition
4. Check the developing unit page counter in Service Mode → Counter. Most bizhub developing units have a rated life of 120,000–200,000 pages depending on model. A developing unit running significantly beyond its rated life will have degraded developer material that produces consistently invalid TCR readings even when the sensor and connectors are in perfect condition
5. If the developing unit is well past its rated life or the developer mix is visibly clumped, contaminated, or discoloured, plan for developing unit replacement regardless of the sensor and connector findings

Step 4 — Perform a TCR Sensor Check in Service Mode

With connectors confirmed good and toner supply confirmed operational, use service mode to directly observe the TCR sensor output and confirm whether the sensor itself is functioning.

1. Enter Service Mode (refer to your model’s service manual for the entry procedure)
2. Navigate to the developing unit / TCR sensor check or I/O monitor section
3. Observe the live TCR sensor reading for the black channel — on most models this is displayed as a numerical value or a voltage reading
4. A healthy black TCR sensor will produce a stable reading within the machine’s specified range (typically displayed in the service manual alongside the sensor check procedure)
5. A reading of 0, maximum value, or a value that fluctuates wildly without any change to the developing unit conditions indicates a failed sensor
6. Also run the ATDC adjustment (automatic toner density control adjustment) if available in service mode — on some models this can recalibrate the sensor reading after a developer disturbance and clear C2564 without replacing any parts

Step 5 — Replace the Black Developing Unit

If the connector path is confirmed good, toner supply is confirmed operational, and the TCR sensor reading is invalid or out of range, the developing unit must be replaced. The TCR sensor is integrated into the developing unit on all bizhub models and is not available as a standalone replacement part.

1. Order the correct black developing unit for your model — part numbers differ between model families, and colour/mono units are not interchangeable
2. Power OFF and unplug the machine before starting
3. Remove the existing black developing unit and set it aside
4. Install the new developing unit — push it fully into its rails until the connector seats completely and the locking latch engages
5. Power on and immediately navigate to Service Mode → Maintenance and reset the developing unit counter for the black channel — failing to reset this counter will cause the machine to continue treating the new unit as if it is at end of life
6. Run a gradation adjustment / ATDC adjustment to allow the machine to calibrate the new unit’s TCR baseline
7. Run a test print — confirm C2564 does not return and that black density is correct and even across the page

Step 6 — Replace the Front Relay Board (FRB / FREYB / PHRYB)

If a new developing unit with a confirmed-good connector still produces C2564, the fault is in the relay board that passes the TCR signal between the developing unit and the main control board. Replace the appropriate relay board for your model:

• FRB — for bizhub C224/C284/C364 series, C224e/C284e/C364e/C454e/C554e, C258/C308/C368, C458/C558/C658, C454/C554
• FREYB — for bizhub C220/C280/C360
• PHRYB — for bizhub C654/C754, 654/754
• TCRSB — for bizhub 363/423 mono models

After board replacement, rerun the ATDC adjustment and a test print to confirm correct TCR sensor reading through the new board.

Step 7 — Check ICP (Inline Circuit Protector) Continuity

On several model families, the PRCB or MFPB has an inline circuit protector (ICP) in the TCR signal path that can blow due to a transient overcurrent event. A blown ICP completely disables the TCR sensor input on the board without any other visible board damage.

• bizhub C224 / C284 / C364 series — check PRCB ICP conduction (refer to service manual for specific ICP location)
• bizhub C258 / C308 / C368 series — check MFPB ICP2E conduction
• bizhub C654 / C754 — check PRCB ICP 17 conduction

Test ICP continuity with a multimeter set to continuity mode. An open-circuit ICP (no continuity) confirms a blown protector. ICPs are surface-mount components — board-level repair is required or the entire PRCB/MFPB must be replaced.

Step 8 — Replace the PRCB or MFPB

If a confirmed-good developing unit, confirmed-good relay board, and intact ICP continuity still produce C2564, the main control board has a failed TCR sensor input circuit. Replace the appropriate board:

• PRCB (Printer Control Board) — for C224/C284/C364 series, C220/C280/C360, C454/C554, C654/C754, and mono models
• MFPB (MFP Board) — for C258/C308/C368 and C458/C558/C658 series

After board replacement, run a full ATDC adjustment and gradation calibration before returning the machine to service. PRCB and MFPB replacement is expensive and should only be performed after all previous steps have been exhausted with confirmed-good parts.

Quick Reference — Troubleshooting by Symptom

Preventing C2564 From Recurring

• Use genuine Konica Minolta toner — compatible toners with incorrect particle sizes and flow characteristics are a leading cause of developer mix degradation and toner supply irregularities that trigger C2564
• Replace developing units on schedule — running past the rated page life degrades the carrier bead charge capacity, causing the ATDC system to progressively lose control of toner concentration until C2564 is triggered
• Inspect developing unit connectors during every PM visit — a 60-second connector inspection and reseat during routine maintenance eliminates the most common cause of C2564 before it becomes a service call
• Run gradation adjustment after every consumable change — this keeps the ATDC baseline calibrated and reduces the risk of false alarms from a newly installed developing unit that has not yet been calibrated to the machine
• Store developing units correctly — developing units stored vertically or exposed to temperature extremes can suffer developer mix separation that causes out-of-range TCR readings immediately on installation

Understanding the C256x Error Code Family

C2564 belongs to a family of four related TCR sensor error codes. If you are seeing multiple codes from this family simultaneously, the fault is almost certainly in a shared component — the relay board or main control board — rather than multiple simultaneous developing unit failures:

• C2561 — Cyan TCR sensor failure (developing unit CN14 / FREYB CN6)
• C2562 — Magenta TCR sensor failure (developing unit CN15 / FREYB CN8)
• C2563 — Yellow TCR sensor failure (developing unit CN16 / FREYB CN10)
• C2564 — Black TCR sensor failure (developing unit CN13 / FREYB CN4)

If two or more of these codes appear together, reseat the relay board’s connection to the main control board first — a single loose connector between the FRB and the PRCB can cause all four channels to fail simultaneously.

Professional Technician Summary

Error Code C2564 on Konica Minolta bizhub machines is a black TCR sensor alarm — and in the overwhelming majority of field cases it is caused by a developing unit that is not fully seated or a connector with poor contact, not a failed sensor or a board problem. The TCR sensor itself almost never fails in isolation without a preceding developer condition issue or a physical connector problem.

Start with the developing unit reinstall. If that does not clear it, work through the connector path systematically using the model-specific references in this guide. Only move to developing unit replacement after confirming the connector path is clean and intact and the toner supply is functioning correctly. Reserve relay board and control board replacement for cases where all consumable and connector causes have been definitively ruled out.