GM Service Manual Online
For 1990-2009 cars only
AFO Enable Circuit Status: Display shows OK, OPEN, SHORT TO GND or SHORT TO VOLTS. This display indicates the monitored status of the output driver circuit for this output. OK should always be indicated unless a circuit fault is monitored.

AFO Signal: Display shows PCM or AFECM. This display monitors the vehicles powertrain control module (PCM) Alternative Fuels Operation (AFO) Signal circuit. Alternative fuel operation is requested when the alternative fuels ECM grounds the AFO Signal circuit to the PCM. This display is received through serial data from the PCM.

AFO Signal Command: Display shows PCM or AFECM. This display monitors the alternative fuels ECM AFO Command circuit output driver. Alternative fuel operation is requested when the AF ECM grounds the AFO Signal circuit to the PCM.

Alternative Fuel Level: Display shows fuel level from 0% to 100%. This display indicates the fuel level signal being sent by the alternative fuels ECM. The alternative fuels ECM monitors the fuel tank pressure sensor and the fuel tank temperature sensor values in order to determine the usable CNG fuel level.

Alternative Fuel Mileage Log: Display shows miles the vehicle has operated using alternative fuel.

(Alt. Fuel) Fuel Pump Relay Command: Display shows ON or OFF. The display indicates the commanded state of the AF Fuel Pump relay. The gasoline fuel pump relay power circuit is routed through the AF Fuel Pump relay. Whenever the engine is operating on CNG the alternative fuels ECM commands the AF Fuel pump relay ON, opening the gasoline fuel pump relay power circuit. The alternative fuels ECM opens the gasoline fuel pump relay power circuit in order to prevent unnecessary gasoline fuel pump operation.

Alternative Fuel Run Time Log: Display shows Hours: minutes: seconds. This display shows the amount of time the engine has operated using alternative fuel.

BARO: Display shows the BARO in Kpa from 11 to 105. The BARO reading is taken at key ON using the MAP sensor and is also corrected at wide open throttle (WOT). Multiply your current, local, barometric pressure by 3.386 to obtain a BARO value that should be within 2 Kpa of the displayed value.

Battery Voltage: Display shows voltage from 0 to 25.5. This display indicates the system voltage measured by the alternative fuels ECM at the battery feed terminal.

ECT Sensor: Display shows the temperature of the engine coolant from -40° to 155°C (-40° to 311°F) and the sensor voltage from 0.0 to 5.00.

EGR Position Sensor: Display shows the position of the EGR valve sensor voltage from 0.0 to 5.00.

Engine Load: Display shows engine load in percent from 0 to 100. Engine load is calculated by the alternative fuels ECM by engine speed and manifold absolute pressure. Engine load should increase with an increase in RPM and MAP.

Engine Run Time: Display shows Hours: minutes: seconds. Engine run time measures how long the engine has been running during this ignition cycle. When the engine is turned OFF, the timer is reset to zero.

Engine Speed: Display shows crankshaft rotation speed in revolutions per minute (RPM) from 0 to 16384. The maximum engine speed is limited to 5500 RPM by the alternative fuels ECM during alternative fuel operation.

EVAP Purge Solenoid Command: Display shows percent. This display monitors the vehicles powertrain control module (PCM) EVAP Purge Solenoid Control circuit.

Fuel Gauge Circuit Status: Display shows OK, OPEN, SHORT TO GND or SHORT TO VOLTS. This display indicates the monitored status of the output driver circuit for this output. OK should always be indicated unless a circuit fault is monitored.

Fuel Gauge Select Switch: Display shows Depressed or Released. This display monitors the vehicles powertrain control module (PCM) Fuel Gauge Select Switch signal circuit. When depressed, the fuel gauge select switch will command the PCM to display the fuel level of the non-operating fuel system for about 10 seconds. This display is received through serial data from the PCM.

Fuel Indicator Lamp Command: Display shows ON or OFF. This display monitors the vehicles powertrain control module (PCM) Fuel Indicator Lamp (FIL) control circuit. The FIL is commanded ON when the engine is operating on gasoline. This display is received through serial data from the PCM.

Fuel Rail Pressure Sensor: Display shows voltage from 0.0 to 5.0 and pressure from 0 to 772 kPa (0 to 112 psi). This display indicates the CNG fuel rail pressure. The alternative fuels ECM monitors the fuel rail pressure (FRP) sensor in order to calculate the correct fuel injector pulse width.

Fuel Rail Temperature Sensor: Display shows voltage from 0.0 to 5.0 and temperature from -40°C to 150°C (-40°F to 302°F). This display indicates the temperature of the CNG fuel within the fuel rail. The alternative fuels ECM monitors the fuel rail temperature (FRT) sensor in order to calculate the correct fuel injector pulse width.

Fuel Tank Pressure Sensor: Display shows voltage from 0.0 to 5.0 and pressure from 0 to 4064 psi. This display indicates the CNG fuel tank pressure. The alternative fuels ECM monitors the fuel tank pressure (FTP) sensor and the fuel tank temperature (FTT) sensor in order to calculate the correct CNG fuel level remaining in the tank.

Fuel Temperature Sensor: Display shows voltage from 0.0 to 5.0 and temperature from -40°C to 150°C (-40°F to 302°F). This display indicates the temperature of the CNG fuel within the fuel tank. The alternative fuels ECM monitors the fuel tank pressure (FTP) sensor and the fuel tank temperature (FTT) sensor in order to calculate the correct CNG fuel level remaining in the tank. The FTT sensor is a non-serviceable part of the high pressure lock-off solenoid.

Gasoline Fuel Mileage Log: Display shows miles the vehicle has operated using gasoline fuel.

Gasoline Fuel Run Time Log: Display shows Hours: minutes: seconds. This display shows the amount of time the engine has operated using gasoline fuel.

High Pressure Lockoff Solenoid Command: Display shows FUEL FLOW or NO FUEL FLOW. This display indicates the commanded state of the high pressure lock-off solenoid. The alternative fuels ECM opens the HPL for one second at EVERY ignition ON in order to supply a prime pulse to the CNG fuel system.

HO2S Bank 1 Sensor 1: Display shows voltage from 0.0 to 1.1 volts. This display indicates the voltage of the O2 sensor High signal circuit. With a COLD O2 sensor the signal circuit BIAS voltage of about 451 mV should be displayed. The oxygen sensor will produce voltage in relation to exhaust oxygen content once the oxygen sensor reaches operating temperature. High exhaust oxygen content, or LEAN conditions, will produce low voltage - 100 mV. Low exhaust oxygen content, or RICH conditions, will produce high voltage -- 900 mV. With the oxygen sensor at operating temperature and the engine in Closed Loop operation the oxygen sensor voltage should rapidly fluctuate between low and high voltage.

HO2S Bank 2 Sensor 1: Display shows voltage from 0.0 to 1.1 volts. This display indicates the voltage of the O2 sensor High signal circuit. With a COLD O2 sensor the signal circuit BIAS voltage of about 451 mV should be displayed. The oxygen sensor will produce voltage in relation to exhaust oxygen content once the oxygen sensor reaches operating temperature. High exhaust oxygen content, or LEAN conditions, will produce low voltage - 100 mV. Low exhaust oxygen content, or RICH conditions, will produce high voltage -- 900 mV. With the oxygen sensor at operating temperature and the engine in Closed Loop operation the oxygen sensor voltage should rapidly fluctuate between low and high voltage.

HPL Solenoid Output Circuit Status: Display shows OK, OPEN, SHORT TO GND or SHORT TO VOLTS. This display indicates the monitored status of the output driver circuit for this output. OK should always be indicated unless a circuit fault is monitored.

IAT Sensor: Display shows the temperature of the intake manifold air from -40° to 155°C (-40° to 311°F) and the sensor voltage from 0.0 to 5.00.

Ignition 0: Display shows voltage from 0 to 25.5. This display indicates the system voltage measured by the alternative fuels ECM at the Ignition 0 terminal.

Injector PWM Average Bank 1: Display shows pulse width in milliseconds. This display indicates the average ON time that the alternative fuels ECM is commanding the bank 1 CNG fuel injectors.

Injector PWM Average Bank 2: Display shows pulse width in milliseconds. This display indicates the average ON time that the alternative fuels ECM is commanding the bank 2 CNG fuel injectors.

Long Term FT Bank 1: Display shows percentage from -100% to 100%. The center position for the long term fuel trim is 0%. The long term fuel trim is based on short term fuel trim learned values. It is stored in memory blocks of the long term memory according to MAP and RPM values. A percentage greater than 0% indicates that time is being added to the injector pulse width, increasing the amount of fuel to the engine. A percentage less than 0% indicates time is being subtracted from the injector pulse width, reducing the amount fuel to the engine. A value of 0% indicates no compensation is required to operate the engine at the desired 16.8 to 1 air/fuel ratio.

Long Term FT Bank 2: Display shows percentage from -100% to 100%. The center position for the long term fuel trim is 0%. The long term fuel trim is based on short term fuel trim learned values. It is stored in memory blocks of the long term memory according to MAP and RPM values. A percentage greater than 0% indicates that time is being added to the injector pulse width, increasing the amount of fuel to the engine. A percentage less than 0% indicates time is being subtracted from the injector pulse width, reducing the amount fuel to the engine. A value of 0% indicates no compensation is required to operate the engine at the desired 16.8 to 1 air/fuel ratio.

Loop Status: Display shows OPEN LOOP or CLOSED LOOP. Closed Loop displayed indicates that the alternative fuels ECM is controlling fuel delivery according to oxygen sensor voltage. In Open Loop, the alternative fuels ECM ignores the oxygen sensor voltage and bases the amount of fuel to be delivered on other input sensors. At extremely high ambient temperatures or when towing a trailer, it is possible for the system to remain in Open Loop operation in order to control catalytic converter temperatures.

Low Pressure Lockoff Solenoid Command: Display shows FUEL FLOW or NO FUEL FLOW. This display indicates the commanded state of the low pressure lock-off solenoid. During alternative fuel operation conditions the LPL remains closed until at least 25 RPM is monitored by the alternative fuels ECM.

LPL Solenoid Output Circuit Status: Display shows OK, OPEN, SHORT TO GND, SHORT TO VOLTS or OVER TEMP. This display indicates the monitored status of the output driver circuit for this output. OK should always be indicated unless a circuit fault is monitored.

MAF Sensor: Display shows the amount of airflow entering the engine in grams per second (g/s) from 0 to 512 and in frequency from 0 to 31,999 Hz. The MAF sensor produces a frequency signal equal to 2.4 g/s whenever the ignition is ON and no airflow is present. The 2.4 g/s signal is necessary in order to detect circuit faults.

MAP Sensor: Display shows manifold pressure in Kilo Pascals (Kpa) from 10 to 104 and voltage from 0 to 5.0. At key ON , engine not running, the MAP value will reflect barometric pressure. To check for an accurate MAP reading multiply your local barometric pressure by 3.386 to obtain a map value that should be within 2 Kpa of displayed value at key ON , engine not running.

(PCM) MIL Command: Display shows ON or OFF. The malfunction indicator lamp (MIL) is controlled by both the alternative fuels ECM and the gasoline powertrain control module (PCM). This display indicates ON whenever the PCM commands the MIL ON. The internal circuitry of the alternative fuels ECM monitors, but does not affect, the PCM operation of the MIL. The MIL is illuminated if either module has detected a problem that affects vehicle emissions, driveabillity or could cause powertrain damage. The PCM illuminates the MIL when the ignition is ON and the engine OFF in order to perform a bulb test.

(Alt. Fuel) MIL Command: Display shows ON or OFF. The malfunction indicator lamp (MIL) is controlled by both the alternative fuels ECM and the gasoline powertrain control module (PCM). This display indicates ON whenever either module commands the MIL ON. The internal circuitry of the alternative fuels ECM monitors, but does not affect, the PCM operation of the MIL. The MIL is illuminated if either module has detected a problem that affects vehicle emissions, driveabillity or could cause powertrain damage. The PCM illuminates the MIL when the ignition is ON and the engine OFF in order to perform a bulb test.

MIL Output CKT Status: Display shows OK, OPEN, SHORT TO GND, SHORT TO VOLTS or OVER TEMP. This display indicates the monitored status of the output driver circuit for this output. OK should always be indicated unless a circuit fault is monitored.

Reference Voltage High: Display shows voltage from 0.0 to 5.2 volts. This display indicates the voltage monitored by the alternative fuels ECM from the gasoline PCM 5 volt sensor supply circuit.

Reference Voltage Low: Display shows voltage from 0.0 to 5.2 volts. This display indicates the voltage monitored by the alternative fuels ECM from the gasoline PCM 5 volt sensor ground circuit.

Short Term FT Bank 1:  Display shows percentage from -100% to 99%. The center position for the short term fuel trim is 0%. A percentage greater than 0% indicates that time is being added to the injector pulse width, increasing the amount of fuel to the engine to compensate for a lean condition sensed by the Bank 1 oxygen sensor. A percentage less than 0% indicates time is being subtracted from the injector pulse width reducing the amount of fuel to the engine to compensate for a rich condition sensed by the Bank 1 oxygen sensor. A value of 0% indicates no compensation is required to operate the engine at the desired 16.8 to 1 air/fuel ratio.

Short Term FT Bank 2:  Display shows percentage from -100% to 99%. The center position for the short term fuel trim is 0%. A percentage greater than 0% indicates that time is being added to the injector pulse width, increasing the amount of fuel to the engine to compensate for a lean condition sensed by the Bank 2 oxygen sensor. A percentage less than 0% indicates time is being subtracted from the injector pulse width reducing the amount of fuel to the engine to compensate for a rich condition sensed by the Bank 2 oxygen sensor. A value of 0% indicates no compensation is required to operate the engine at the desired 16.8 to 1 air/fuel ratio.

TP Sensor: Display shows throttle angle from 0 to 100% and sensor voltage from 0.0 to 5.0 volts. The % display indicates the throttle angle opening. Throttle closed is 0% and fully open is 100%. The alternative fuels ECM learns the closed throttle position whenever the TP sensor voltage is below about 0.9 volts. Learning the closed throttle position is necessary in order to adjust for production tolerances. The voltage display indicates the throttle angle opening.Throttle closed is about 0.5 volts and fully open is about 4.0 volts.

TR Switch: Display shows the position of the trans range (TR) switch.

Vehicle Speed Sensor: Display shows vehicle speed from 0 km/h (0 mph) to 255 km/h (155 mph). The maximum vehicle speed is limited to 107 mph by the alternative fuels ECM during alternative fuel operation.