Takagi TK-Jr2 Tankless Error Codes

The Takagi TK-Jr2 is a popular residential natural gas or propane tankless water heater with a 140,000 BTU input rating, capable of producing 5.8 GPM of hot water. It’s known for reliability, but when a fault code appears on the digital display, the numeric codes can seem cryptic. Unlike other brands that use letter codes (E1, E2), Takagi uses a 2-digit numeric fault code system with distinct codes for every failure mode.

This guide is the most complete reference available for TK-Jr2 fault codes - covering all 15 primary codes, what each one means at the component level, and how to diagnose and fix each one. Whether you’re a homeowner running a basic check or an HVAC/plumbing technician on a service call, this covers everything.

What Does Takagi TK-Jr2 Error Codes Mean?

The TK-Jr2 control board monitors the entire operating cycle: flow detection, pre-purge, ignition sequence, flame verification, water temperature, combustion air, and heat exchanger temperature. When the board detects an out-of-spec condition, it shuts the unit down, displays the fault code, and stores it in the fault history.

The TK-Jr2’s fault history can hold the last 3 fault codes. To retrieve the fault history, press and hold the “Set” button on the remote control for 5 seconds - the display cycles through the last 3 stored codes. This is invaluable for diagnosing intermittent faults.

Takagi TK-Jr2 Complete Fault Code Reference

Code 11 - Ignition Failure The unit attempted to ignite 3 times and did not detect a flame signal within the trial-for-ignition period.

Causes:

• Failed igniter (Part # 100013730 for TK-Jr2 compatible igniter assembly)
• Dirty or failed flame rod sensor
• Low gas pressure (should be 3.5-10.5 inches w.c. natural gas; 8.0-14.0 inches w.c. propane)
• Air in the gas line (initial installation or after supply interruption)
• Closed or partially open gas supply valve

Code 12 - Flame Loss During Operation The unit ignited but the flame sensor lost the signal during a hot water draw.

Causes:

• Dirty flame sensor rod - the most common cause
• Gas pressure drop during operation (another large gas appliance activating)
• Failed gas valve (intermittent closure)
• Combustion air problem in a confined installation

Code 14 - Thermal Fuse Blown A thermal fuse on the gas valve or heat exchanger assembly has opened. This is a one-shot safety device. Unlike a resettable high-limit switch, the thermal fuse must be replaced once it trips.

Causes:

• Dry fire (unit operated without adequate water flow)
• Combustion gas recirculation overheating the unit body
• Failed temperature sensor causing over-firing
• Part number for Takagi TK-Jr2 thermal fuse: TK-Jr2-THERMO (compatible: 10A 250V 139°C rated thermal fuse)

Code 16 - Outlet Water Temperature Too High The outlet thermistor detected water temperature exceeding 185°F (85°C).

Causes:

• Very low flow rate with maximum temperature setpoint
• Failed thermistor reading low (causing board to over-fire)
• Scale buildup in heat exchanger creating hot spots
• Failed gas valve stuck partially open at full gas rate

Code 31 - Inlet Water Temperature Sensor Failure The cold water inlet temperature sensor (NTC thermistor) is reading outside its normal range.

Causes:

• Failed inlet thermistor (open or shorted)
• Loose connector on control board
• Wiring damage between sensor and board

Code 32 - Outlet Water Temperature Sensor Failure The hot water outlet temperature sensor is reporting an out-of-range value.

Causes:

• Failed outlet thermistor
• Scale encasing the sensor reducing thermal contact
• Loose or corroded connector

Code 33 - Combustion Air Temperature Sensor Failure The combustion air temperature sensor (used by the TK-Jr2 to monitor intake air) is faulty.

Causes:

• Failed combustion air sensor
• Connector damage

Code 52 - Modulating Gas Valve Fault The proportional modulating gas valve is not responding correctly to control signals. The TK-Jr2 uses a modulating gas valve that adjusts BTU output based on demand. This is a higher-value component than a simple on/off valve.

Causes:

• Failed gas valve modulating coil
• Control board failure (gas valve command circuit)
• Wiring fault between board and valve
• Valve stuck in fixed position (mechanical failure)

Code 61 - Fan Motor Failure The combustion air fan is not running or not reaching commanded speed. The TK-Jr2 uses fan speed monitoring (tachometer signal) to verify safe combustion air flow.

Causes:

• Failed fan motor (Part # 100013732 for TK-Jr2 compatible fan)
• Blocked flue or air intake
• Fan blade obstructed by debris
• Tachometer signal wire failure

Code 65 - Water Flow Control Valve Fault The water flow control valve (water volume control) is not responding correctly. The TK-Jr2 has an internal water control valve that regulates flow to prevent thermal shock and modulate output.

Causes:

• Failed water control valve
• Debris in valve mechanism
• Failed valve control motor
• Wiring fault

Code 72 - Flame Signal Without Gas Valve Command The flame sensor is detecting a flame when the control board has not commanded the gas valve to open. This is a gas valve safety fault.

Causes:

• Leaking gas valve (gas passing through closed valve)
• Faulty flame sensor generating false signal
• Electrical interference on the flame sense circuit

⚠️ Code 72 indicates a potential gas leak through the valve. If this code appears, verify no gas odor before restarting. Call a licensed plumber to inspect the gas valve.

Code 76 - Communication Error Between Controller and Unit The remote controller (if installed) has lost communication with the main unit.

Causes:

• Damaged communication wiring between remote and unit
• Failed remote controller
• Failed control board communication circuit
• Wiring polarity reversed during installation

Code 91 - Exhaust Overtemperature The exhaust temperature sensor detected flue gas temperatures above the safe limit. This indicates incomplete heat transfer - the combustion gases are leaving the unit too hot.

Causes:

• Scale buildup in heat exchanger severely reducing thermal transfer
• Heat exchanger partially blocked
• Incorrect gas pressure (over-pressurized, resulting in excess combustion)
• Cold climate condensate drain blockage affecting flue

Code 93 - Condensate Neutralizer / Drain Fault (Condensing Models) On condensing TK-Jr2 models, the condensate system has a fault.

Causes:

• Blocked condensate drain
• Frozen condensate line in cold weather
• Failed condensate pump (if installed)
• Failed drain monitoring switch

Code 99 - False Flame Signal at Startup / Fan Preignition Error Before the ignition sequence starts, the board checks that no flame is present. Code 99 means the flame sensor detected a signal before ignition - suggesting a false reading or a genuinely abnormal pre-existing flame.

Causes:

• Dirty flame sensor generating a false signal at room temperature
• Flame sensor wire shorted to chassis
• Failed control board

How to Fix It

Step 1: Retrieve fault history before resetting. Press and hold the Set button on the TK-Jr2 remote for 5 seconds. Record all 3 stored codes. A sequence like Code 11 → Code 12 → Code 11 tells you the unit attempted ignition, briefly achieved flame, then lost it repeatedly - pointing to a gas supply or flame sensor issue rather than a complete igniter failure.

Step 2: For Code 11 (ignition failure), test igniter and flame sensor first. Shut off gas and power. Access the burner chamber (remove the front cover, typically 4 screws). The igniter assembly is a spark electrode positioned approximately 3-5mm from the grounding surface. Check for visible damage - a cracked ceramic insulator means replacement is needed. The igniter part # for the TK-Jr2 is 100013730 (or compatible cross-reference). For the flame sensor, clean the rod tip with steel wool and re-test.

Step 3: For Code 12, check gas pressure at the manifold. While the unit is running (if it can sustain a brief run), measure gas manifold pressure with a U-tube manometer or digital pressure gauge at the test port on the gas valve. For natural gas, manifold pressure at maximum firing should be 3.0-3.5 inches w.c. If pressure drops below 2.5 inches w.c. during operation, the issue is upstream gas supply, not the unit itself.

Step 4: For Code 14 (thermal fuse), find the root cause before replacing. The thermal fuse (located on the gas valve or heat exchanger body, depending on exact assembly year) blew for a reason. Check:

• Were there any Code 31/32 faults before Code 14? A failed temperature sensor could have caused over-firing.
• Is the heat exchanger heavily scaled? Scale reduces water flow, raising heat exchanger body temperature. Replace the thermal fuse (generic 139°C 10A 250V fuse from any electronics supplier), and then address the root cause.

Step 5: For Code 52 (gas valve fault), test the modulating coil. The modulating gas valve on the TK-Jr2 has a proportional coil that the control board drives with a variable current. Disconnect the valve wiring connector and measure resistance across the modulating coil terminals. Expected resistance: 25-35 ohms for typical Takagi modulating valves. An open circuit confirms coil failure. A reading within range suggests the fault is in the board’s drive circuit.

Step 6: For Code 61 (fan failure), inspect flue first. Remove the flue pipe from the unit and check for obstructions at the termination cap. Also inspect the air intake (many TK-Jr2 models have a direct-vent configuration with separate intake and exhaust). Spin the fan blade by hand - it should turn freely. Any resistance suggests bearing failure. If the fan spins freely and the flue is clear but Code 61 persists, the fan motor itself needs replacement (Part # 100013732 or equivalent).

Step 7: For Code 91 (exhaust overtemperature), perform heat exchanger service. Code 91 on a TK-Jr2 that’s been in service for 3+ years in a hard water area almost always means significant scale on the heat exchanger. The descaling procedure: isolate the water supply at the service valves (most TK-Jr2 installations include a flushing port), connect a submersible pump to the cold inlet service port, and circulate 3 gallons of undiluted white vinegar for 45 minutes. Flush with clean water. Vinegar descaling resolves Code 91 in the majority of cases without heat exchanger replacement.

Parts You May Need

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When to Call a Pro

The Takagi TK-Jr2 is a professional-grade appliance, and several faults require licensed technician involvement:

• Code 52 (gas valve): Modulating gas valve replacement requires leak testing and manifold pressure adjustment after installation.
• Code 72 (flame without command): This is a potential gas leak situation. Have a licensed plumber inspect the valve before any restart.
• Code 14 (thermal fuse) with suspected heat exchanger damage: If descaling doesn’t resolve underlying overtemperature conditions, heat exchanger replacement requires a plumber.
• Code 91 that returns within 6 months of descaling: This suggests the heat exchanger is failing and approaching end of life - get a professional evaluation.
• Any time you smell gas: Do not reset the unit. Shut off gas, ventilate, and call the gas company.

Frequently Asked Questions

Q: My TK-Jr2 shows Code 11 only on the first call for hot water each day, then works fine after one reset. What causes this? A: Morning ignition failures that clear after a reset usually point to one of two things: (1) overnight condensation on the flame sensor rod - the moisture briefly disrupts the flame signal on first ignition, then burns off; (2) air that slowly migrates into the gas line overnight, requiring one purge cycle to clear. Clean the flame sensor rod thoroughly and see if the morning Code 11 disappears.

Q: Code 12 appears when I run the dishwasher and the shower simultaneously. Is the TK-Jr2 undersized? A: Not necessarily undersized, but likely experiencing a gas pressure drop. Running multiple gas appliances simultaneously (dishwasher’s gas dryer nearby, gas range, etc.) can temporarily drop manifold pressure below the Code 12 threshold. Have a plumber measure dynamic gas pressure (with all appliances running) at the meter and at the TK-Jr2 inlet. If pressure drops significantly under load, a larger gas supply line may be needed.

Q: The Code 91 keeps coming back 3 months after descaling. Is it time to replace the unit? A: Code 91 returning quickly after descaling suggests either very hard water is re-scaling the unit rapidly (consider a water softener or a whole-house scale inhibitor), or the heat exchanger has physical damage - corroded fins, a partial blockage that vinegar can’t dissolve, or a collapsed tube. A plumber can perform a visual inspection after removal. If the unit is 10+ years old, replacement is often more economical than heat exchanger replacement.

Q: How do I access the fault history if I don’t have the remote controller? A: On the TK-Jr2, the remote controller is standard equipment but may have been misplaced. Takagi sells replacement remotes. In the absence of a remote, fault codes are only accessible on the display as the current active code. If the fault code clears before you see it, have a plumber connect a diagnostic tool, or call Takagi technical support (1-888-882-5244) for guidance on reading fault memory from the board directly.

Q: Can I use a generic NTC thermistor to replace the TK-Jr2 inlet sensor? A: Yes, with the correct specifications. The TK-Jr2 uses 10kΩ NTC thermistors with a standard B-constant. Measure the resistance of your failed sensor at room temperature - a good one reads approximately 10,000 ohms at 77°F. Match the replacement to the same connector type and mounting bracket. Many generic HVAC thermistors with 2-pin connectors are direct replacements and cost significantly less than OEM parts.