Scan Tool Data Definitions

A list of explanations for each data message displayed on the scan tool begins below. This information will assist you in tracking down emission or driveability problems since the displays can be viewed while the vehicle is being driven. Refer to the for additional information.

Engine Speed: The scan tool range is 0-3175 RPM. Engine speed is computed by the ECM from the ignition reference pulses. It should remain close to desired idle under various engine loads with engine idling.

Desired Idle Speed: The scan tool range is 0-3175 RPM. The desired idle is the idle speed that is commanded by the ECM. The ECM will compensate for various engine loads based to keep the engine at the desired idle speed.

ECT: The scan tool range is 40°C-199°C (-40°F-389°F). The Engine Coolant Temperature (ECT) sensor is mounted in the intake manifold and sends engine temperature information to the ECM.The ECM supplies 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 ECM 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 ECM will interpret the lower voltage as a warm engine.

MAP Sensor: The scan tool range is 10-205 kPa, 0.00-5.10 volts. The manifold absolute pressure (MAP) sensor measures the change in the intake manifold pressure from engine load 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 for the following: (1) To update the BARO reading; (2) Enabling factor for several of the diagnostics.

Throttle Position: The Throttle Position (TP) sensor produces a voltage proportional to the position of the throttle valve. It is used to indicate to the ECM how much throttle is being commanded by the driver. A reading close to 5.10 volts means full throttle, while a reading of 0.00 volts corresponds to idle throttle.

Throttle Angle: Computed by the ECM from the TP sensor voltage, throttle position should read 0% at idle and 100% at wide open throttle.

O2S: The scan tool range is 0-1800 mV. The scan tool display represents the exhaust Oxygen sensor (02S) output voltage. The voltage should fluctuate constantly within a range between 100 mV (lean exhaust), and 1000 mV (rich exhaust), while operating in closed loop.

Loop Status: This position will indicate whether the control module is operating in Open or Closed Loop. Most systems go Closed Loop after a certain amount of run time, when engine coolant temperature is high enough, and the oxygen sensors becomes active.

Rich/Lean Status: Scan tool displays Rich or Lean. Indicates whether the exhaust oxygen sensor voltage is above (rich) or below (lean) the 450 mV oxygen threshold voltage. The display should change constantly indicating that the ECM is controlling the air/fuel mixture properly.

O2S Cross Counts: The scan tool range is 0-255. This is the number of times the O2S voltage crosses over the rich/lean threshold during a one second interval.

KS Activity: The scan tool displays Yes or No. The display indicates whether or not a KS activity is being detected by the ECM. With the engine idling, the scan tool should indicate No at idle..

Long Term FT Bank : The scan tool range is 0-255. Block learn is derived from the fuel intergrator value and is used for long-term correction of fuel delivery. A value of 128 counts indicates that the fuel delivery requires no compensation to maintain a 14.7:1 air/fuel ratio. A value below 128 counts means that the fuel system is too rich and fuel delivery is being reduced (decreased injector pulse width). A value above 128 counts indicates that a lean condition exists and the ECM is compensating by adding fuel (increased injector pulse width).

Long Term FT Bank : The scan tool displays percentage. 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%/128 indicates that fuel delivery requires no compensation to maintain the ECM commanded air/fuel ratio. A negative value significantly below 0%/128 indicates that the fuel system is rich and fuel delivery is being reduced (decreased injector pulse width). A positive value significantly greater than 0%/128 indicates that a lean condition exists and the ECM is compensating by adding fuel (increased injector pulse width).

Short Term FT Bank : The scan tool range is 0-255.Fuel intergrator represents a short term correction to fuel delivery by the ECM 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 then 450 mV, indicating a lean air/fuel mixture. Fuel intergrator will increase and the ECM will add fuel. If the oxygen sensor voltage stays mainly above the threshold, the ECM reduces fuel delivery to compensate for the indicated rich condition.

Short Term FT Bank: The scan tool displays percentage. Short term fuel trim represents a short term correction to fuel delivery by the ECM in response to the amount of time the fuel control oxygen sensor voltage spends above or below the 450mV threshold. If the oxygen sensor voltage has mainly remained less than 450mV, indicating a lean air/fuel mixture,short term fuel trim will increase into the positive range above 0%/128 and the ECM will add fuel. If the oxygen sensor voltage stays mainly above the threshold, short term fuel trim will decrease below 0%/128 into the negative range while the ECM 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. Fuel trim values at maximum authority may indicate an excessively rich or lean system.

Idle Air Control: The scan tool range is 0-255. The scan tool displays the commanded position of the idle air control pintle in counts. The higher the number of counts, the greater the commanded idle speed. Idle air control should respond fairly quickly to changes in engine load to maintain desired idle RPM.

PNP: The scan tool displays R-D321, or neutral. Medium duty trucks do not have a park gear. When the transmission is in neutral, the scan tool will read NEUTRAL . When it is in gear it will read R-D321.

System Voltage Ignition 1: The scan tool range is 0-25.5 volts. This represents the system voltage measured by the ECM at its ignition feed.

Fuel Pump Volts: The scan tool range is 0-25.5 volts. This represents the voltage found at the fuel pump. It is used by the ECM as the system voltage. The ECM uses fuel pump volts as a reference to determine electrical loading.

A/C Clutch: The air conditioner clutch is a feedback signal to the ECM that lets the ECM know if the A/C clutch is ON or OFF.

Gov. Overspeed Lamp.: The scan tool reads OFF or ON. For medium duty trucks there is an engine overspeed governor that keeps the engine from over revving. When the ECM recognizes that the engine is spinning too fast, it turns ON a dash light and this parameter will read ON.

Vehicle Speed: The scan tool displays 0-255 km/h (0-158 mph). The vehicle speed sensor signal is converted into km/h and mph for display.

PROM ID: This parameter describes the particular PROM used in the ECM being tested. The PROM contains the ECM program. Calibration ID is used when its necessary to replace the PROM. Calibration ID must be specified when ordering new PROM(s). Calibration ID should not be confused with the part number.