GM Service Manual Online
For 1990-2009 cars only
3X Crank Sensor: Scan Tool Range 0-9999. Displays engine speed.

24X Crank Sensor: Scan Tool Range 0 to approximately 1600 RPM. Displays the engine speed calculated from the 24X Reference signal PCM input. Should nearly match ENGINE SPEED up to approximately 1,600 RPM limit.

Abuse Management: Scan Tool Displays ACTIVE or INACTIVE. ACTIVE indicates the PCM is commanding protection against abnormal driveline conditions. For further information refer to Automatic Transaxle Diagnosis.

A/C High side: Scan Tool Displays 0 to 459 kPa. Represents the A/C refrigerant pressure sensor signal. The amount of pressure indicates the amount of load that the A/C compressor is placing on the engine. The PCM uses this information to adjust idle and to control the cooling fans.

A/C OFF For WOT: Scan Tool Displays YES or NO. Yes displayed indicates that the PCM has commanded the A/C compressor clutch relay offcause the throttle position is greater than the threshold for A/C compressor operation (TP Angle greater than 90%).

A/C Pressure Disable: Scan Tool Displays YES or NO. YES displayed indicates that the PCM is monitoring an A/C Refrigerant Pressure signal voltage which is too high or too low to allow the A/C compressor clutch to engage.

A/C Request: Scan Tool Displays YES or NO. Indicates the state of the A/C request input circuit from the HVAC controls. The PCM uses the A/C request signal to determine whether A/C compressor operation is being requested.

Actual EGR Position: Scan Tool Range 0% to 100%. Represents the actual position of the EGR Pintle in percent. 0% displayed indicates a fully extended pintle (EGR valve closed).

Air Fuel Ratio: Scan Tool Range 0.0 to 25.5. Air Fuel Ratio indicates the PCM commanded value. In closed loop, Air Fuel Ratio should normally be around 14.2-14.7. A lower Air Fuel Ratio indicates a richer commanded mixture, which may be seen during Power Enrichment or TWC Protection modes. A higher Air Fuel Ratio indicates a leaner commanded mixture. This can be seen during Decel Fuel Mode.

BARO: Scan Tool Range 10 to 105 kPa/0.00 to 5.00 Volts. The Barometric Pressure reading is determined from the MAP sensor signal monitored during key up and Wide-Open Throttle (WOT) conditions. The Barometric Pressure is used to compensate for altitude differences.

Brake Switch: Scan Tool Displays RELEASED or APPLIED. This input to the PCM represents the state of the vehicle brake pedal. When the brake switch indicates that the brake pedal is APPLIED, the PCM will command the transaxle torque converter clutch to disengage.

Cam Signal Present: Scan Tool Displays YES or NO. Indicates whether or not the correct CAM signal has been received by the PCM during the last six 3X Reference pulses. NO displayed indicates no CAM signal received. A toggling display indicates an intermittent CAM signal.

Commanded A/C: Scan Tool Displays ON or OFF. Represents the PCM commanded state of the A/C compressor clutch relay driver circuit. The A/C compressor clutch should be engaged when Commanded A/C displays ON.

Commanded Fuel Pump: Scan Tool Displays ON or OFF. Indicates the PCM commanded state of the fuel pump relay driver circuit. The fuel pump will be run at high speed to deliver more fuel volume when fuel flow or MAP is greater than a certain level, or when system voltage is less than 10 volts.

Commanded Gen: Scan Tool Displays ON or OFF. OFF displayed indicates that the PCM has commanded the generator L terminal OFF.

Cruise Mode:  Scan Tool Displays yes or no. Represents the state of the Cruise Status Input form the SMCC module. CRUISE ENGAGED should not display YES unless the vehicle is operating in a cruise control mode.

Cruise Inhibited: Scan Tool Displays yes or no. YES displayed indicates that the PCM is commanding the SMCC module to inhibit cruise control operation. Should display YES at idle.

Current Gear: Scan Tool Displays numbers 1, 2, 3, or 4. The PCM looks at Trans Range position and the gear commanded by the PCM. The actual forward gear that the Transaxle is in will be the lower of the two because the manual valve can override the PCM commanded gear.

Decel Fuel Mode: Scan Tool Displays ACTIVE or INACTIVE. ACTIVE displayed indicates that the PCM has detected conditions appropriate to operate in Deceleration Fuel Mode. The PCM will command deceleration fuel mode when a sudden decrease in throttle position has been detected while the vehicle is traveling over 25 mph. While in Deceleration Fuel mode, the PCM will decrease the amount of fuel delivered by entering Open Loop and decreasing the injector pulse width.

Desired EGR Position: Scan Tool Range 0% to 100%. Represents the EGR pintle position that the PCM is commanding. Desired EGR Pos. should stay close to Actual EGR Pos.

Desired Idle Speed: Scan Tool Range 0 to 3,187 RPM. The idle speed that is commanded by the PCM. The PCM will compensate for various engine loads based on engine coolant temperature to keep the engine at the desired speed.

ECT: Scan Tool Range -40°C to 151°C,-40°F to 304°F. The Engine Coolant Temperature (ECT) sensor is mounted in the coolant stream and sends engine temperature information to the PCM. The PCM applies 5 volts to the ECT sensor circuit. The sensor is a thermistor which changes internal resistance as temperature changes. When the sensor is cold (internal resistance high), the PCM monitors a high signal voltage and interprets it as a cold engine. As the sensor warms (internal resistance decreases), the voltage signal will decrease and the PCM will interpret the lower voltage as a warm engine.

EGR Closed Pintle Pos: Scan Tool Range 0.0 to 5.0 Volts. Represents the learned voltage that the PCM uses to determine whether or not the EGR valve is fully closed (0 percent pintle position). If the EGR PWM is 0% and a difference of more than 0.4 volt between EGR Feedback and EGR Closed Valve Pintle Position is detected, DTC P1406 will set.

EGR Duty Cycle: Scan tool Range 0% to 100%. Represents the PCM commanded PWM duty cycle of the Exhaust Gas Recirculation (EGR) valve. 0% displayed indicates no EGR; 100% displayed indicates full EGR.

EGR Feedback: Scan Tool Range 0.00 to 5.00 Volts. Indicates the EGR pintle position sensor signal voltage being monitored by the PCM. A low voltage indicates a fully extended pintle (closed valve), A voltage near 5 volts indicates a fully retracted pintle (open valve).

EGR Position Error: Scan Tool Range 0% to 100%. Indicates the difference between Desired EGR Position and Actual EGR Position currently being monitored by the PCM. If the EGR Position Error is reaches an unacceptable level, DTC P1406 will set.

EGR Flow Test Count: Scan Tool Range 0 to 255. Indicates the number of EGR flow test samples collected during the current ignition cycle. The maximum number of samples allowed is 1 during normal operation. If the PCM battery feed has been disconnected or a DTC P0401 has been cleared, 10 EGR flow test samples will be allowed during the next ignition cycle. This is to allow repair verification during a single ignition cycle.

Engine Load: Scan Tool Range 0% to 100%. Engine load is calculated by the PCM from engine speed and MAF sensor readings. Engine Load should increase with an increase in RPM or air flow.

Engine Oil Level: Scan Tool Displays OK or LOW. Indicates the oil level as determined by the Engine Oil Level sensor.

Engine Run Time: Scan Tool Range 00:00:00 to 99:99:99 Hrs:Min:Sec. Indicates the time elapsed since the engine was started. If the engine is stopped, ENGINE RUN TIME will be reset to 00:00:00.

Engine Speed: Scan Tool Range 0 to 9,999 RPM. Engine speed is computed by the PCM from the 3X reference input. It should remain close to desired idle under various engine loads with engine idling.

EVAP Canister Purge: Scan Tool Range 0% to 100%. Represents the PCM commanded PWM duty cycle of the EVAP Purge Solenoid valve. 0% displayed indicates no purge; 100% displayed indicates full purge.

EVAP Vent Solenoid: Scan Tool Displays Open or Closed.

Low Speed Fans, High Speed Fans: (if applicable) Scan Tool Displays ON or OFF. Represents the PCM commanded state of the Low Speed and High Speed fan relay drivers.

Fuel Trim Cell: Scan Tool displays cell#. Fuel trim cell is dependent upon engine speed and MAF sensor readings. A plot of RPM vs. MAF is divided into 10 cells. Fuel trim cell indicates which cell is currently active.

Fuel Trim Learn: Scan Tool Displays ENABLED or DISABLED. When conditions are appropriate for enabling long term fuel trim corrections, FUEL TRIM LEARN will display yes. This indicates that the long term fuel trim is responding to short term fuel trim. If the Fuel Trim Learn displays NO, long term fuel trim will not respond to changes in short term fuel trim.

Generator Lamp: Scan Tool Displays ON or OFF. ON displayed indicates the PCM commanded state of the generator lamp/check gauges lamp due to low system voltage or a generator fault detected.

HO2S 1: Scan Tool Range 0 to 1132 mV. Represents the fuel control exhaust oxygen sensor output voltage. Should fluctuate constantly within a range between 10 mV (lean exhaust) and 1000 mV (rich exhaust) while operating in Closed Loop.

HO2S 1: Scan Tool Displays NOT READY or READY. Indicates the status of the fuel control exhaust oxygen sensor. The scan tool will indicate that the exhaust oxygen sensors are ready when the PCM detects a fluctuating HO2S voltage sufficient to allow closed loop operation. This will not occur unless the exhaust oxygen sensors are warmed up.

HO2S 2: Scan Tool Range 0 to 1,132 mV. Represents the catalyst monitor exhaust oxygen sensor output voltage. If the catalyst is operating efficiently, the HO2S 1 signal will be far more active than that produced by the HO2S 2 sensor. If the PCM detects a level of HO2S 2 activity that indicates the catalyst is no longer operating efficiently, DTC P0420 will be set.

HO2S XCounts: Scan Tool Range 0 to 255. Indicates the number of times the HO2S 1 signal has crossed the 450 mV bias voltage during a 1 second interval. A HO2S 1 signal that almost never crosses the bias voltage could indicate a fuel trim problem, a faulty HO2S 1 circuit, or a faulty HO2S 1.

Hot Mode: Scan Tool Displays Active or Inactive. ON displayed indicates that the PCM has detected conditions appropriate for Transaxle hot mode operation. The PCM determines this base on Transaxle fluid temperature. While in Hot Mode, Transaxle shift points may be altered, 4th gear disabled, and the TCC forced ON in 2nd gear.

IAC Position: Range 0 to 255. Displays the commanded position of the idle air control pintle in counts. A larger number of counts means that more air is being commanded through the idle air passage. IAC Position should respond fairly quickly to changes in engine load to maintain desired idle RPM.

IAT: Scan Tool Range -40°C to 151°C (-40°F to 304°F). The PCM converts the resistance of the intake air temperature sensor to degrees. Intake Air Temperature (IAT) is used by the PCM to adjust fuel delivery and spark timing according to incoming air density.

Ignition Mode: Scan Tool displays Bypass/IC. Bypass indicates the ignition control module is keeping the spark advance at a fixed 10 degrees BTDC (Bypass Mode). The ignition control module determines the correct operating mode based on the level of voltage that the PCM sends to the ignition control module on the Bypass circuit. If the PCM does not turn ON the 5 volts, or if the ignition control module doesn't receive it, the module will control spark timing. IC indicates that the PCM has signaled the ignition module that the PCM is going to control the spark advance (IC mode). The ignition control module determines the correct operating mode based on the level of voltage that the PCM sends to the ignition control module on the Bypass circuit. The PCM provides 5 volts to the ignition control module Bypass circuit if the PCM is going to control spark timing (IC mode). If the PCM does not turn ON the 5 volts, or if the ignition control module doesn't receive it, the module will control of spark timing (Bypass mode).

Injector Pulse Width: Scan Tool Range 0 to 1000 msec. Indicates the amount of time the PCM is commanding each injector ON during each engine cycle. A longer injector pulse width will cause more fuel to be delivered. Inj. Pulse Width should increase with increased engine load.

Knock Retard: Scan Tool Range 0.0 degrees to 25.5 degrees. Indicates the amount of spark the PCM is removing from IC spark advance in response to the signal from the knock sensors. The Traction Control system Desired Torque signal from the EBTCM may cause Knock Retard to display a value greater than 0.0 degrees if Traction Control is active.

Long Term FT: Scan Tool Range -23% to 16%. Long Term Fuel Trim is derived from the Short Term Fuel Trim value and represents a long-term correction of fuel delivery. A value of 0% indicates that fuel delivery requires no compensation to maintain the PCM commanded air/fuel ratio. A negative value significantly below 0% indicates that the fuel system is rich and fuel delivery is being reduced (decreased injector pulse width). A positive value significantly greater than 0% indicates that a lean condition exists and the PCM is compensating by adding fuel (increased injector pulse width). Because Long Term Fuel Trim tends to follow Short Term Fuel Trim; a value in the negative range due to canister purge at idle should not be considered unusual. The PCM s maximum authority to control long term fuel trim allows a range between -23% and +16%. Fuel trim values at maximum authority may indicate an excessively rich or lean system.

Loop Status: Scan Tool Displays OPEN or CLOSED. Closed Loop indicates that the PCM is controlling fuel delivery according to oxygen sensor voltage. In Open Loop, the PCM ignores the oxygen sensor voltage and bases the amount of fuel to be delivered on TP sensor, engine coolant, and MAF sensor inputs only.

Low Oil Lamp: (if applicable) Scan Tool Displays ON or OFF Indicates the state of the Low Oil Level Indicator based ON the Engine Oil Level switch input to the PCM.

MAF: Scan Tool Range 0.0-512 g/s. MAF (Mass Air Flow) is the MAF Input Frequency converted to grams of air per second. This indicates the amount of air entering the engine.

MAF Frequency: Scan Tool Range 0 to 32000 Hz. The Mass Air Flow sensor input signal to the PCM is a frequency which varies from around 3000 Hz at idle to above 7000 Hz at wide-open throttle. The PCM converts this frequency signal to grams per second to be displayed as Mass Air Flow.

MAP: Scan Tool Range 10-105 kPa/0.00 to 4.97 Volts. The Manifold Absolute Pressure (MAP) sensor measures the change in the intake manifold pressure from engine load, EGR flow, and speed changes. As intake manifold pressure increases, intake vacuum decreases resulting in a higher MAP sensor voltage and kPa reading. The MAP sensor signal is used to monitor intake manifold pressure changes during the EGR flow test, to update the BARO reading, and as an enabling factor for several of the diagnostics.

MIL: Scan Tool Displays ON or OFF. Indicates the PCM commanded state of the Malfunction Indicator Lamp.

Total Misfire Failures: Scan Tool Range 0 to 65535 Counts. Indicates the number of 200 crankshaft revolution sample periods during which the level of misfire was sufficiently high to report a failure.

Misfire Current Cyl #1 / Misfire Current Cyl #2 / Misfire Current Cyl #3 / Misfire Current Cyl #4 / Misfire Current Cyl #5 / Misfire Current Cyl #6: Scan Tool Range 0 to 198 Counts. The Misfire Current counters indicate the number of cylinder firing events detected as possible misfires on each cylinder during the last 200 crankshaft revolutions. The number that the counters display are twice the actual number of misfire events detected. The counters may normally display some activity, but the activity should be very low and nearly equal for all the cylinders.

Misfire History Cyl #1 / Misfire History Cyl #2 / Misfire History Cyl #3 / Misfire History Cyl #4 / Misfire History Cyl #5 / Misfire History Cyl #6: Scan Tool Range 0 to 65,535 Counts. The Misfire History counters indicate the total level of misfire that has been detected on each cylinder. The number that the counters display are twice the actual number of misfire events detected. The Misfire History counters will not update or show any activity until a misfire DTC P0300 has become active. The Misfire History counters will be updated every time a 200 crankshaft revolution misfire test failure is detected.

Total Misfire Passes Since First Fail: Scan Tool Range 0 to 65,535 Counts. Indicates the number of 200 crankshaft revolution sample periods during which the level of misfire was sufficiently low to report a pass.

Non Volatile Memory: Scan Tool Displays PASS or FAIL. Indicates the condition of the PCM s random access memory.

Power Enrichment: Scan Tool Displays ACTIVE or INACTIVE. ACTIVE displayed indicates that the PCM has detected conditions appropriate to operate in Power Enrichment mode. The PCM will command Power Enrichment mode when a large increase in throttle position and load is detected. While in Power Enrichment, the PCM will increase the amount of fuel delivered by entering Open Loop and increasing the injector pulse width. This is done to prevent a possible sag or hesitation from occurring during acceleration.

Short Term FT: Scan Tool Range -10% to 10%. Short Term Fuel Trim represents a short-term correction to fuel delivery by the PCM in response to the amount of time the fuel control oxygen sensor voltage spends above or below the 450 mV threshold. If the oxygen sensor voltage has mainly remained less than 450 mV, indicating a lean air/fuel mixture, short term fuel trim will increase into the positive range above 0% and the PCM will add fuel. If the oxygen sensor voltage stays mainly above the threshold, short term fuel trim will decrease below 0% into the negative range while the PCM reduces fuel delivery to compensate for the indicated rich condition. Under certain conditions such as extended idle and high ambient temperatures, canister purge may cause Short Term Fuel Trim to read in the negative range during normal operation. The PCM s maximum authority to control fuel trim allows a range between -11% and +20%. Fuel trim values at maximum authority may indicate an excessively rich or lean system.

Spark: Scan Tool Range -64 degrees to 64 degrees. Displays the spark timing being commanded by the PCM on the IC circuit. A negative (-) value indicates degrees Before Top Dead Center (BTDC) or spark advance; a positive value indicates degrees After Top Dead Center (ATDC) or spark retard. Because the ignition control module sets spark advance at a fixed value of 10 degrees BTDC while operating in Bypass mode, the displayed Spark advance only reflects true spark timing when the PCM has commanded IC mode.

Start-Up Ect: Scan Tool Range -40°C to 151°C (-40°F to 304°F). Indicates the Engine Coolant Temperature at the time that the vehicle was started. Used by the HO2S diagnostic to determine if the last startup was a cold start.

Start-Up IAT: Scan Tool Range -40°C to 151°C (-40°F to 304°F). Indicates the Intake Air Temperature at the time that the vehicle was started. Used by the HO2S diagnostic to determine if the last startup was a cold start.

TCC Engaged: Scan Tool Displays Engage or Disengage. Indicates the commanded state of the Transaxle Torque Converter Clutch.

Total Misfire Current Count: Scan Tool Range 0 to 99. Indicates the total number of cylinder firing events that were detected as being misfires during the last 200 crankshaft revolution sample period.

TP Angle: Scan Tool Range 0% to 100%. TP Angle is computed by the PCM from the TP Sensor voltage. TP Angle should display 0% at idle and 100% at wide open throttle.

TP Sensor: Scan Tool Range 0.00 to 5.00 Volts. The voltage being monitored by the PCM on the TP sensor signal circuit.

Transmission Range: Scan Tool Displays PARK/NEUTRAL, REVERSE, DRIVE 4, DRIVE 3, DRIVE 2, or LOW. Trans Range represents the decoded status of the four inputs from the Transaxle Range Switch Assembly. The combination of Transaxle range inputs indicates the position of the Transaxle manual valve.

TR Switch P A B C: Scan Tool Displays Low or High. The TR P A B C display represents the states of the four inputs from the Transaxle Range Pressure Switch Assembly. High indicates a battery positive voltage signal being monitored at the PCM, Low indicates a grounded input signal. The combination of Transaxle range inputs indicates the position of the Transaxle manual valve.

Vehicle Speed: Scan Tool Range 0-255 km/h, 0-155 mph. The vehicle speed sensor signal is converted into mph and km/h for display.

VTD Fuel Disable: Scan Tool Displays ACTIVE or INACTIVE. (if appliable) The Theft Deterrent Fuel Enable circuit is an input from the Vehicle Theft Deterrent Control Module that signals the PCM to enable the injectors if the proper signal is received. The scan tool will normally display INACTIVE. If the Vehicle Theft Deterrent Control Module does not sent the correct Theft Deterrent Fuel Enable signal to the PCM, the display will switch to ACTIVE and the fuel system may be disabled. Refer to Vehicle Theft Deterrent diagnosis for further information.