Circuit Description
The fuel level sensor changes resistance based on fuel level. The fuel level sensor has a signal circuit and a ground circuit. The powertrain control module (PCM) applies a voltage, about 5 volts, on the signal circuit to the sensor. The PCM monitors changes in this voltage caused by changes in the resistance of the sensor to determine fuel level.
When the fuel tank is full, the sensor resistance is high, and the PCM signal voltage is only pulled down a small amount through the sensor to ground. Therefore, the PCM will sense a high signal voltage, that is, the fuel tank is full. When the fuel tank is empty, the sensor resistance is low, and the signal voltage is pulled down a greater amount. This causes the PCM to sense a low signal voltage, meaning that the fuel tank is empty.
The PCM uses inputs from the fuel level sensor in order to calculate the total fuel remaining in the fuel tank. The PCM uses the fuel level information for the evaporative emission (EVAP) and misfire diagnostics. The fuel level information is sent to the instrument panel cluster (IPC) via the Class 2 circuit. The IPC controls the fuel gauge based on the Class 2 message. The IPC also displays a LOW FUEL message when the fuel level is below a predetermined level or when a fuel level sensor DTC sets.
This DTC sets when the PCM senses a signal voltage higher than the normal operating range of the sensor.
Conditions for Running the DTC
The engine is operating.
Conditions for Setting the DTC
| • | The fuel level sensor voltage is above 2.9 volts. |
| • | The condition is present for 2 minutes. |
Action Taken When the DTC Sets
| • | The PCM stores the DTC information into memory when the diagnostic runs and fails. |
| • | The malfunction indicator lamp (MIL) will not illuminate. |
| • | The PCM records the operating conditions at the time the diagnostic fails. The PCM stores this information in the Failure Records. |
| • | The message center displays the LOW FUEL message. |
| • | The fuel gauge displays empty. |
Conditions for Clearing the DTC
| • | A last test failed, or current DTC, clears when the diagnostic runs and does not fail. |
| • | A history DTC will clear after 40 consecutive warm-up cycles, if no failures are reported by this or any other non-emission related diagnostic. |
| • | Use a scan tool in order to clear the DTC. |
Diagnostic Aids
Important: Remove any debris from the PCM connector surfaces before servicing the PCM. Inspect the PCM connector gaskets when diagnosing or replacing the PCM. Ensure that the gaskets are installed correctly. The gaskets prevent water intrusion into the PCM.
Depending on the current fuel level, it may be difficult to locate a malfunctioning sending unit. The malfunction may only occur when the fuel level is full or near empty. The fuel sending unit may need to be removed for further diagnosis. A fuel level sensor that has an intermittent condition causes this DTC to set. Remove the fuel level sensor. Refer to Fuel Level Sensor Replacement in order to test the resistance of the sensor. The resistance of the sensor should change smoothly from 40-250 ohms. Replace the sensor if the resistance did not change smoothly or was out of range.
The following occurs with this DTC set:
| • | The vehicle fuel gauge displays empty. |
| • | The message center displays a message. |
The PCM will not reset the mileage for this diagnostic if the fuel tank is re-fueled to the same level as before the repair was made. Perform the following procedure in order to re-fuel the vehicle after replacing a sending unit or fuel tank:
- Add 25 percent of the fuel that you removed before the repair.
- Start and idle the engine for 5 minutes.
- Turn OFF the engine.
- Add the remainder of the fuel.
Using Freeze Frame and Failure Records data may aid in locating an intermittent condition. If you cannot duplicate the DTC, the information included in the Freeze Frame and Failure Records data can aid in determining how many miles since the DTC set. The Fail Counter and Pass Counter can also aid in determining how many ignition cycles the diagnostic reported a pass or a fail. Operate the vehicle within the same Freeze Frame conditions, such as RPM, load, vehicle speed, temperature etc., that you observed. This will isolate when the DTC failed.
For an intermittent, refer to Symptoms .
Test Description
The numbers below refer to the step numbers on the diagnostic table.
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If DTC P1635 and P1639 set at the same time, this indicates that the signal circuit is shorted to a voltage.
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This step determines if the fault is present.
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The fuel level sensor may only fail in a certain range. If you determine the condition is intermittent, inquire with the customer when the concern occurred. The concern may only happen when the vehicle is refueled or when the fuel level is close to empty.
Using the Freeze Frame and Failure Records data may aid in locating an intermittent condition. If you cannot duplicate the DTC, the information included in the Freeze Frame and Failure Records data can help determine how many miles since the DTC set. The Fail Counter and Pass Counter can also help determine how many ignition cycles the diagnostic reported a pass or a fail. Operate the vehicle within the same freeze frame conditions, such as RPM, load, vehicle speed, temperature etc., that you observed. This will isolate when the DTC failed. For an intermittent condition, refer to Symptoms .
-
If the voltage goes low, this determines the circuits and the PCM are OK.
-
Inspect for poor connections at the PCM harness before replacing the PCM.
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
1 | Did you perform the Powertrain On-Board Diagnostic (OBD) System Check ? | -- | ||
Did DTC P1635 and P1639 also Fail This Ignition cycle? | -- | |||
Is the fuel level sensor voltage above the specified value? | 2.9 V | |||
Does the scan tool indicate that this diagnostic failed this ignition? | -- | Go to Diagnostic Aids | ||
Is the fuel level sensor voltage below the specified value? | 2.9 V | |||
6 | Jumper the fuel level sensor signal circuit to a known good ground. Is the fuel level sensor voltage below the specified value? | 2.9 V | ||
7 |
Is the fuel level sensor signal circuit open? | -- | ||
8 |
Does the DMM display a resistance within the specified range? | 0-2 ohms | ||
9 |
Is the fuel level sensor ground circuit open? | -- | ||
10 |
Did you find and correct the condition? | -- | ||
11 | Replace the fuel level sensor. Refer to Fuel Level Sensor Replacement . Is the action complete? | -- | -- | |
12 | Repair the open fuel level sensor signal circuit. Refer to Wiring Repairs in Wiring Systems. Is the action complete? | -- | -- | |
13 | Repair the open fuel level sensor ground circuit. Refer to Wiring Repairs in Wiring Systems. Is the action complete? | -- | -- | |
14 | Repair the fuel level sensor signal circuit for a short to voltage. Refer to Wiring Repairs in Wiring Systems. Is the action complete? | -- | -- | |
15 |
Did you find and correct the condition? | -- | ||
|
Important: The replacement PCM must be programmed. Replace the PCM. Refer to Powertrain Control Module Replacement . Is the action complete? | -- | -- | ||
17 |
Does the scan tool indicate that this test ran and passed? | -- | ||
18 | Select the Capture Info option and the Review Info option using the scan tool. Does the scan tool display any DTCs that you have not diagnosed? | -- | Go to the applicable DTC table | System OK |
