Diesel Heater Error Codes: Complete Troubleshooting Guide

September 29, 2026

Your diesel heater is showing an error code — here is exactly what it means and how to fix it.

This guide covers every fault code for BISON Series air heaters (2000A, 2200A, 4000A, 5000A) and WALRUS Series coolant heaters (5000C, 9000C).

How to read your code:

Rotary controller (BISON only) — count the number of flashes before the pause. Match the number to the error group below.
Digital timer controller — the two-character hex code (e.g. E3, 50) is displayed directly on screen.

BISON Series Error Codes (2000A, 2200A, 4000A, 5000A)

Reference values — check these before replacing any parts:

Supply voltage: 12V unit: 10.5–16V / 24V unit: 21.5–30V
Glow pin resistance: 12V: 0.1–0.2Ω / 24V: 0.7–1.2Ω
Temperature sensor resistance: 1.1kΩ at room temperature

Flashes	Hex Code	Error	Fix
—	00	No connection	Check power supply, harness connection, and controller wiring
1	10	Startup failed (normal mode)	Check air inlet/outlet, fuel level, fuel line, glow pin resistance. Clean carbon deposits. Replace ECU if all else fails
1	12	Startup failed (increased oxygen mode)	Same as code 10
2	20	Flameout (normal mode)	Check for air in fuel line, test fuel pump output, clean carbon deposits, replace burner
2	24	Flameout (increased oxygen mode)	Same as code 20
3	30	Voltage too high	Measure supply voltage. Check alternator and battery connections
3	31	Voltage too low	Charge battery. Check wiring connections for corrosion
4	40	Flame at self-inspection stage	Heater still hot — turn off, allow to cool, restart
4	41	Furnace too hot before ignition	Use ventilation mode to cool. Check temp sensor (1.1kΩ). Replace ECU if sensor is OK
4	42	Furnace overheated	Same as code 41
5	50	Flame sensor open circuit	Test circuit continuity. Replace flame sensor. Replace ECU if sensor is OK
5	51	Flame sensor short circuit	Check for frayed wiring. Replace flame sensor or ECU
5	52	Supply temp sensor open circuit	Test continuity. Replace temp sensor. Replace ECU if sensor is OK
5	53	Supply temp sensor short circuit	Check for frayed wiring. Replace sensor or ECU
6	65	Inlet temp sensor open circuit	Test continuity. Replace temp sensor. Replace ECU if sensor is OK
6	66	Inlet temp sensor short circuit	Check wiring. Replace sensor or ECU
6	67	Intake air temperature too high	Check outlet air is not recirculating into inlet. Cool down and restart
6	68	External temp sensor open circuit	Test continuity. Replace temp sensor. Replace ECU if sensor is OK
6	69	External temp sensor short circuit	Check wiring. Replace sensor or ECU
7	70	Fuel pump open circuit	Check pump connector pins and harness. Replace fuel pump. Replace ECU if pump is OK
7	71	Fuel pump short circuit	Same as code 70
8	80	Blower motor open circuit	Test motor and circuit. Replace blower motor. Replace ECU if motor is OK
8	81	Blower motor short circuit	Check wiring. Replace blower. Replace ECU if blower is OK
8	82	Fan speed too slow	Check voltage and ducting. Replace ECU if both are OK
8	83	Fan speed too fast	Check voltage and ducting. Replace ECU if both are OK
8	84	Fan speed measurement failure	Check air impeller for damage. Replace ECU if impeller is OK
8	85	Fan failed to start	Check fuses and wiring. Check if fan is stuck. Replace blower
9	90	Glow pin open circuit	Check voltage, test glow pin resistance, clean carbon deposits. Replace glow pin. Replace ECU if pin is OK
9	91	Glow pin short circuit	Same as code 90
9	92	Wrong type or broken glow pin	Confirm correct glow pin for your voltage (12V vs 24V). Replace glow pin
9	93	Glow pin driver open circuit	Same as code 90
10	A2	Furnace too hot during heating	Check inlet temp is below 35°C. Confirm inlet/outlet not blocked. Check outlet air not recirculating
10	A4	Inlet air too hot before ignition	Same as A2
10	A9	Abnormal power loss	Check power supply. Replace ECU
11	B4	Furnace temp sensor open circuit	Check sensor pins. Replace temp sensor (1.1kΩ normal). Replace ECU if sensor is OK
11	B5	Furnace temp sensor short circuit	Same as B4
—	C0	Vehicle heater open circuit	Check vehicle electrical wiring
—	C1	Vehicle heater short circuit	Check vehicle electrical wiring
—	C4	Controller open circuit	Replace the controller
—	C5	Controller short circuit	Replace the controller
—	D0	ECU failure	Replace the ECU
—	D1	Failed to save information	Replace the ECU
—	D3	Maintenance warning	Clean combustion chamber, replace burner and wearable parts
—	E0	Cannot detect ignition signal	Replace the ECU
—	E1	No power at glow pin driver	Replace the ECU
—	E2	Glow pin monitoring failed	Replace the ECU
—	E3	No signal from flame sensor	Confirm flame sensor is plugged in. Replace flame sensor. Replace ECU if sensor is OK
—	EE	Unknown fault	Replace the ECU

WALRUS Series Error Codes (5000C, 9000C)

Reference values:

Supply voltage: 12V unit: 10.5–16V / 24V unit: 21.5–30V
Flame sensor resistance: 0.8kΩ at room temperature
Temperature sensor resistance: 10kΩ at room temperature

Hex Code	Error	Fix
00	No connection	Check power supply, harness connection, and controller wiring
10	Voltage too high	Check power supply and connections
11	Voltage too low	Check power supply. Charge battery
12	Coolant overheat — software limit	Check coolant level. Refill after cooldown. Check coolant pump
13	Second failure	Check fuel level, fuel line, air intake and exhaust
14	Coolant temp too high / temp difference too large	Check coolant level. Check coolant pump
15	Overheat lock (10 times)	Check coolant level. Check coolant pump
17	Coolant overheat — hardware limit	Check coolant level. Check coolant pump
20	Glow pin open circuit	Clean carbon from glow pin. Replace glow pin. Replace ECU if pin is OK
21	Glow pin short circuit	Same as code 20
22	Wrong type or glow pin performance lost	Confirm correct glow pin is installed. Replace glow pin
30	Fan speed too fast	Replace ECU
31	Fan circuit open	Check blower fan mounting. Replace blower motor. Replace ECU if motor is OK
32	Fan circuit shorted	Check wiring. Replace blower. Replace ECU if blower is OK
33	Fan speed too slow	Check voltage. Check blower fan mounting. Replace ECU if both are OK
38	Warm air blower short circuit	Check fuses and wiring
39	Warm air blower open circuit	Check fuses and wiring
41	Coolant pump open circuit	Check coolant pump line. Replace coolant pump
42	Coolant pump short circuit	Check coolant pump line. Replace coolant pump
45	Fuel heater short circuit	Check fuses and wiring
46	Fuel heater open circuit	Check fuses and wiring
47	Fuel pump short circuit	Check ECU-to-pump connection. Replace fuel pump. Replace ECU if pump is OK
48	Fuel pump open circuit	Same as code 47
50	Start failure lock (10+ attempts)	Check fuel level, fuel line, air intake and exhaust. Resolve underlying fault before restarting
51	Flame sensor too hot at startup	Ventilate and cool down. Check flame sensor resistance (0.8kΩ)
52	Flameout 3 times	Check fuel level, fuel line, air intake and exhaust
55	Flame detected before ignition	Ventilate and cool down. Check flame sensor resistance (0.8kΩ)
60	Temperature sensor open circuit	Check resistance (10kΩ normal). Replace temp sensor
61	Temperature sensor short circuit	Same as code 60
62	Coolant temp too high before ignition	Cool down coolant. Check temp sensor resistance (10kΩ). Replace temp sensor
64	Flame sensor open circuit	Check resistance (0.8kΩ). Replace flame sensor
65	Flame sensor short circuit	Same as code 64
71	Overheat sensor open circuit	Check overheat sensor (power-off protection switch). Replace sensor
72	Overheat sensor short circuit	Same as code 71
84	Blower fan speed monitoring failed	Check air impeller for damage. Replace ECU if impeller is OK
85	Blower fan failed to start	Check fuses and wiring. Check if fan is stuck
86	Circulation fan short circuit	Check fuses and wiring
91	ECU controller failure	Check heater and timer connections. Confirm sufficient voltage
99	Failed to save information	Replace the ECU
A9	Abnormal power loss	Check power supply. Replace ECU
AA	Unknown glow pin test error	Replace glow pin. Replace ECU if fault continues
AB	Unknown blower motor test error	Replace blower motor. Replace ECU if fault continues
AC	Unknown voltage test error	Check supply voltage and fuses. Replace ECU
AD	Unknown fuel pump test error	Check ECU-to-pump connection. Check fuel line. Replace fuel pump. Replace ECU
AE	Unknown fuel pump failure	Replace fuel pump. Replace ECU if fault continues
AF	Unknown temp sensor failure	Replace temp sensor. Replace ECU if fault continues
B0	Communication failure	Check all ECU connections. Replace ECU
D3	Maintenance warning	Clean carbon deposits, replace burner and consumables
E0	Ignition signal not detected	Check fuses and wiring. Replace ECU
E1	Glow pin driver voltage not detected	Check glow pin installation. Check fuses. Replace ECU
E2	Glow pin monitoring circuit failure	Check fuses and wiring. Replace ECU
E3	Flame sensor signal not detected	Check resistance (0.8kΩ). Replace flame sensor. Replace ECU if sensor is OK
EE	Unknown error	Replace the ECU

Replace the Part First — Not the ECU

The ECU is the most expensive component in a diesel heater. Always replace the specific part indicated first — a flame sensor costs $46, an ECU costs far more. Only replace the ECU if the fault persists after replacing the indicated component and confirming all wiring is undamaged.