Diagnostic Instructions

DIAGNOSTIC INSTRUCTIONS
The Natural Gas Vehicle (NGV) module monitors the components and circuitry required for fuel indication during normal operation. If an error or concern is detected, the NGV module will store a Diagnostic Trouble Code (DTC) in memory. DTCs are retrieved from the NGV module using the scan tool. If a concern exists with an instrument cluster component that is not related to the fuel indication system, refer to the symptom chart in Instrument Panel, Gauges and Warning Indicators for diagnosis of that component. DO NOT perform NGV module diagnostics. Diagnosis By Symptom - Natural Gas - Instrument Cluster

The components monitored by the NGV module include:
^ the tank temperature sensor
^ the tank pressure sensor
^ the fuel gauge
^ the circuitry

Fuel gauge accuracy can be affected by certain conditions. These conditions will cause abnormal fuel gauge movement, possibly causing unnecessary diagnostics. Make sure the following conditions do not exist before continuing with fuel indication system diagnostics:

A. Fast Filling
- After fast filling a vehicle, the fuel gauge reading can drop as much as 1/16 of a tank. This is caused by the rapid expansion and compression of the fuel during fast fills. This is considered normal gauge movement.
- Fast filling with the ignition switch in the RUN position can cause the NGV module to store DTCs in error. With the ignition switch in the RUN position, the NGV module is monitoring the sensors. During fast fill situations, the tank temperature drops to extremely low temperatures; this may cause the tank temperature sensor to drop below its operating temperature range, causing the NGV module to store a DTC. When fast filling is complete, the tank temperature sensor will return to normal operation after it reaches normal operating temperature range. However, the DTC will still be stored.
- Fast filling with the ignition switch in the RUN position will cause the fuel gauge to move very slowly toward Full; this is eliminated when the vehicle is filled with the ignition switch in the OFF position.

B. Fuel system leaks
- Fuel leaks can cause inaccurate fuel gauge readings. Depending on the size of the leak, the fuel gauge may not be able to maintain an accurate reading if the fuel leak exceeds the instrument cluster and NGV module's ability to update the fuel gauge. This creates a possibility for the fuel gauge to display a fuel level when the fuel tanks are empty. Before diagnosing this type of concern, make sure the fuel system is not leaking.

NOTE: Refer to Fuel Delivery and Air Induction for the fuel system leak test.

- After service of a fuel system that has been drained, the fuel gauge will display empty during the initial key-on of the ignition switch since there is no pressure in the fuel lines. After the 4-5 key-ons, the fuel gauge will display the appropriate reading.