Fuel Fired Heater Disabled
The PCM has detected an abnormal temperature reading from the fuel fired heating system.
Code Set Parameters
Temperature readings that do not fall within the manufacturer’s predetermined range of degrees, after a certain period of time with the fuel fired heater system activated, will cause a code to be stored and possibly a service engine soon lamp to be illuminated. Some models may require multiple failure cycles in order for a service engine soon lamp to be illuminated.
Symptoms are not likely to be noticed except during cold ambient temperatures. The fuel fired heating system is typically used to provide heat for the passenger compartment, sideview mirrors, seats, and other equipment. The climate control blower may be inoperative because the blower is often the cause of this code being stored (see diagnostic strategy below). Climate control system, body control module, or instrument panel control system codes may also accompany this code.
The most common cause of this type of code can be contributed to blower (fan motor) failure. A system safety feature is built into the fuel fired heater system. This safety feature will disable the fuel fired burner element if the blower is inoperative. System electrical wiring, connectors, and/or sensors may also be the cause of such a code.
Techs report that, because fuel fired heaters are so rare in North America, repairs are often attempted by diagnosing the engine cooling system. Fuel fired heating systems use warm air created by a hot element that is totally isolated from the engine cooling system.
- Some diesel and gasoline powered vehicles utilize the on-board fuel supply to heat the passenger compartment and other components
- The advantage of a fuel fired heating system is that it reaches its maximum temperature much faster than conventional heaters that utilize engine coolant for heat
- Fuel fired heaters are typically supplied with fuel via small hoses or lines from the fuel tank
- When the fuel pump begins to operate, these lines are filled with fuel and, when the heating system is activated by the driver, some type of electric igniter initiates a combustible action that heats an element much like that of a natural gas space heater
- Once the element reaches a certain temperature, the blower fan propels the heated air throughout the passenger compartment
- In most applications, the fan is also used as a ventilation device because the burning fuel creates harmful carbon monoxide
- Other models utilize engine vacuum to draw harmful carbon monoxide vapors from the heater box into the engine where they are burnt as engine fuel
Various safety sensors are used in this type of system to ensure that passengers are not harmed by heater emissions
- These sensors are often at the root of what contributed to the cause of this code being stored in the PCM
- The blower has an integrated sensor that alerts the PCM if it becomes inoperative for any reason, a temperature sensor allows the PCM to monitor heater box temperatures and disable the igniter if necessary
- Some models also utilize an on-board carbon monoxide detector monitored by the PCM
- The PCM disables igniter operation and in some cases the flow of fuel if excessive carbon monoxide is detected in the heater box or passenger compartment
Several tools may be needed to successfully diagnose this code
- A suitable scanner (or code reader), a digital volt/ohmmeter, a manufacturer’s service manual (or the equivalent), and an infrared thermometer with a laser pointer will be most helpful in performing a successful diagnosis
Begin with a visual inspection of all wiring and connectors
- Repair or replace damaged, disconnected, shorted, or corroded wiring, connectors, and components as necessary
- Always retest the system after repairs are completed to ensure success.
If all system wiring, connectors, and components (Including fuses) appear to be in normal working order, connect the scanner (or code reader) to the diagnostic connector and record all stored codes and freeze frame data
- This information can be extremely helpful in diagnosing intermittent conditions that may have contributed to this code being stored
- Continue by clearing the code and operating the vehicle to see if it returns
- This will help to determine whether or not the malfunction is intermittent
After the codes are cleared, test drive the vehicle to see if the code returns
- If the code fails to immediately return, you may have an intermittent condition
- Intermittent conditions can prove to be quite a challenge to diagnose and in extreme cases may have to be allowed to worsen before a correct diagnosis can be made
Proceed with you diagnosis by testing the blower motor for voltage and ground signals
- Place the blower switch on the “high” setting and there should be battery voltage present at the blower motor connector (with a solid ground)
- If you have battery voltage and a ground, and the blower motor fails to operate, replace the blower motor
- Clear the code and operate the system to ensure that the repair was successful.
If no voltage signal is present at the blower motor connector, when the switch is placed on the high setting, suspect a faulty blower speed switch
- Follow manufacturer’s wiring diagrams and specifications to diagnose the blower motor switch
- If the switch values fail to coincide with manufacturer’s specs, replace the switch
- If no voltage is detected at the blower speed control switch, check system fuses, wiring, and fusible links
- Repair or replace wiring and components as required and operate the system to ensure a successful repair
If the blower motor is operating normally, test the signal wire for a reference voltage to the PCM
- You may also use the scanner to activate the blower motor and observe the perception of the PCM regarding its operation
- If the blower motor is operational and the PCM fails to recognize the fact, suspect a defective blower motor sensor or faulty PCM
- PCM failure is rare and replacement will require reprogramming of one or more controllers.
Once proper blower motor operation is verified, test the blower motor, system temperature, and carbon monoxide sensors (where applicable)
- Follow manufacturer’s specifications when testing the sensors and replace them as required
- Clear the code and operate the fuel fired heater to ensure a successful repair
- If your findings comply with manufacturer’s specifications, disconnect the electrical connectors from the respective sensor/s and any related controllers, then perform a continuity and resistance test on all system circuitry
- Repair or replace shorted, open, disconnected, or corroded wiring, connectors, or components as required and retest the system to ensure a successful repair.
If all electrical circuits coincide with manufacturer’s specs, suspect a defective PCM or auxiliary controller
- Controller failure is rare and will likely require reprograming.