Fuel Level SensorB Circ
The PCM has detected a voltage input signal from the fuel level sensor “B” circuit (or gauge) that does not coincide with the actual amount of fuel in the tank. The B circuit denotes a potential area of suspicion more than a particular wire, connector, or component.
Code Set Parameters
The PCM interprets fuel level as increments of voltage. Since the fuel level system utilizes a low voltage signal, the increments of voltage tend to be very small. Typical maximum system voltage is just 5-volts. Fuel gauge voltage readings that vary from the manufacturer’s reference voltage readings (for any given level of fuel present in the tank) will cause a code to be stored and a service engine soon lamp to be illuminated. Voltage readings that exceed maximum projected system voltage will also cause a code to be stored and a service engine soon lamp to be illuminated.
Typical symptoms may include a fluctuating fuel gauge, the fuel gauge may rest well past the full mark or well below the empty indicator, a service engine soon lamp, and a stored trouble code. If the vehicle is equipped with a low fuel warning system, the fuel warning lamp may flash on and off or remain illuminated and a low fuel alarm could sound.
The most common cause of this malfunction is probably a faulty instrument cluster but other possibilities are also feasible. Damage to the fuel tank can interfere with the fuel level sender and cause it to read incorrectly, there may be an open or shorted ground in the fuel level sensor’s ground, the ground wire or strap could have become corroded or loose and ground resistance is excessive, the signal circuit may also be shorted (either to voltage or ground) or broken. The PCM, BCM, or other fuel level controller could also be faulty but this is the least likely cause.
Frequently fuel level sending units and instrument clusters are replaced without a thorough diagnosis of electrical circuitry. After the expensive components are replaced, the problem still exists.
- Most modern vehicles use a fuel level sending unit that consists of some type of a float on an arm that is attached to a series of assorted tiny resistors
- The fuel level sending unit/fuel gauge circuit is made up of a low voltage signal (5-volts is the norm) and a ground signal, although some models are also equipped with a separate control module for fuel supply and fuel level monitoring
- Other models use a rheostat in the instrument panel to buffer the voltage signal and make fuel gauge operation more refined
- Still others use the PCM, IPC, or BCM to monitor fuel system voltage
- Regardless of the system with which your vehicle is equipped, they use a similar theory of operation
- As the float changes position with the fuel level, it moves the arm which has a contact (on the opposite end) that moves along the series of resistors creating more (or less) resistance in the circuit and slightly changing voltage output to the fuel gauge, fuel module, or rheostat
- These voltage variations result in changes in the fuel level gauge that make the driver aware of how much fuel remains in the tank
- The fuel level sending unit is typically integrated into the fuel pump module assembly, which is almost always located inside the fuel tank.
Several tools may be needed to successfully diagnose this code if no exhaust leaks are detected
- A suitable scanner (or code reader), a digital volt/ohmmeter, and a smoke machine 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
The simplest method for testing the fuel gauge is to disconnect the fuel sending unit connector and ground the signal wire
- With the ignition turned to the on position, the fuel gauge should move to one extreme or the other
- Removing the ground from the signal wire and leaving it unplugged should cause the fuel gauge to move to the opposite end of the spectrum
- Some models will drop past empty and others will travel past the full mark
- If the gauge responds as described, the gauge is good and the wiring is also intact
- In this case, the most likely cause of failure is a bad fuel sending unit.
If the signal wire is properly grounded and the gauge remains unchanged, suspect a faulty fuel gauge, instrument panel unit, or a circuit malfunction
- If you suspect that the gauge is faulty, then remove the instrument cluster and carefully inspect the circuit board
- If there are signs of shorted circuits, corrosion, or damage the instrument cluster is faulty
- Replace the fuel gauge or instrument cluster as required.
Begin checking system wiring by doing a voltage drop test between the fuel sending unit ground signal and the battery ground
- Variations between the fuel sending unit ground and the battery ground should not exceed about 100 millivolts (.1 volt)
- If a difference of 1 volt is present, then a malfunction is present
- Removing the fuel tank ground strap and cleaning the surfaces to which it is secured may correct this condition
- Replacing the ground with a new ground wire is also an option
Test system wiring by disconnecting the PCM (or whatever control module with which your vehicle is equipped) and checking for continuity between the appropriate instrument cluster connector, control module connector and the fuel level sending unit connector
- If no continuity is found on either of the circuits, repair the open circuit as required
- If resistance levels are excessive or if voltage and ground signals are discovered where they should not be, then repair the shorted wiring as needed
If everything else checks out, suspect the control module or PCM
- Failure of the fuel level controller is rare and should be considered as a last resort.