Scan Tool Data Definitions

Powertrain Control Module (PCM) DATA DESCRIPTION
A list of explanations for each data message displayed on the Tech 1 Scan tool begins below. This information will assist in tracking down emission or driveability problems, since the displays can be viewed while the vehicle is being driven. See the "Diagnostic System Check" for additional information.

TYPICAL ENGINE SPEED - Range 0-999 RPM - Engine speed is computed by the PCM from the fuel control reference input. It should remain close to desired idle under various engine loads with engine idling.

DESIRED IDLE - A/T 700-800, M/T 800-900 - The idle speed that is commanded by the PCM. The PCM will compensate for various engine loads to keep the engine at the desired idle speed.

ENGINE COOLANT TEMP (ECT)- Range -40 to 199°C, -40 to 389°F - The Engine Coolant Temperature Sensor is mounted in the intake manifold and sends engine temperature information to the PCM. The PCM supplies 5 volts to the engine coolant temperature 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 which it interprets 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.

MAP - Range Volts at IDLE - 1.04-1.83, 4-4.8 volts at Wide Open Throttle (WOT). The Manifold Absolute Pressure (MAP) sensor measures the changes in the intake manifold pressure which results from engine load and RPM changes and converts these into a voltage output.

THROT POSITION (TP)- Range 0-5.10 Volts - Used by the PCM to determine the amount of throttle demanded by the driver. Should read 0.25-0.98 volt at idle to above 4 volts at wide open throttle.

THROTTLE ANGLE - Range 0-100% - Computed by the PCM from TP sensor voltage should read 0% at idle, 100% at Wide Open Throttle (WOT). This will not indicate 100% until engine is running.

O2 SENSOR - Range 0-1800 mV - Represents the exhaust oxygen sensor output voltage. Should fluctuate constantly within a range between 100 mV (lean exhaust) and 1000 mV (Rich exhaust) when operating in "Closed Loop."

LOOP STATUS - Tech 1 scan tool Displays "Open" or "Closed" - "Closed Loop" displayed 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, coolant temperature, and MAP sensor inputs only. "Closed Loop" engine operation should begin when the O2 sensor becomes active, engine coolant temperature exceeds 65°C (149°F) for more than 30 seconds.

RICH/LEAN STATUS - Tech 1 scan tool Displays RICH or LEAN Indicates whether exhaust oxygen sensor voltage is above (rich) or below (lean) the 450 mV oxygen sensor threshold voltage. Should change constantly indicating that the PCM is controlling the air/fuel mixture properly.

O2S CROSS COUNTS - Range 0-255 - The number of times the oxygen sensor voltage crosses over the rich/lean threshold during a one second interval.

S.T. FUEL TRIM - Range 0-255 - S.T. Fuel Trim represents a short-term correction to fuel delivery by the PCM in response to the amount of time the oxygen sensor voltage spends above or below the 450 mV threshold. If the oxygen sensor voltage has mainly been below 450 mV, indicating a lean air/fuel mixture S.T. Fuel Trim will increase to tell the PCM to add fuel. If the oxygen sensor voltage stays mainly above the threshold, the PCM will reduce fuel delivery to compensate for the indicated rich condition.

L.T. FUEL TRIM - Range 0-255 - L.T. Fuel Trim is derived from the L.T. Fuel Trim value and is used for long-term correction of fuel delivery. A value of 128 counts indicates that 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 PCM is compensating by adding fuel (increased injector pulse width). L.T. Fuel Trim tends to follow S.T. Fuel Trim; values of 110-138 counts at idle should be considered normal. Under certain conditions such as extended idle and high ambient temperatures, EVAP canister purge may cause L.T. Fuel Trim to read as low as 70 counts.

IDLE AIR CONTROL POS - Range 0-255 - Displays the commanded position of the Idle Air Control (IAC) pintle in counts. The higher the number of counts, the greater the commanded idle speed. Idle Air control (IAC) should respond fairly quickly to changes in engine load to maintain desired idle RPM.

PARK/NEUTRAL POS - Tech 1 scan tool Displays "P/N" or "RDL" - "P/N" displayed indicates that the gear select lever is in "Park" or "Neutral."

VEHICLE SPEED - Range 0-255 kph, 0-159 MPH - The Vehicle Speed Sensor (VSS) signal is converted into kph and mph for display.

TCC MODE - Tech 1 scan tool displays TCC "OFF" or "ON" when commanded by the PCM.

TCC NC - Tech 1 scan tool displays "High" or "Low." This input determines that the A/C has been selected and the PCM should compensate for additional load by engaging the TCC at a higher vehicle speed.

KNOCK SIGNAL - Tech 1 scan tool Displays "NO" or "YES" - Indicates whether or not a knock signal is being detected by the PCM. Should read "NO" at idle.

FUEL PUMP VOLTS - Range 13.5-14.5 Volts - When the ignition switch is turned to the "ON" position, the PCM turns the Fuel Pump Relay "ON" position, the PCM turns the Fuel Pump Relay "ON" for two seconds. If the PCM does not receive ignition reference pulses within two seconds, the PCM will shut "OFF" the Fuel Pump.

SYSTEM Volts - Range 0-25.5 Volts - This represents the system voltage measured by the PCM at its ignition #1 feed.

FAN 1 - When the PCM is commanding Fan 1 "ON," the Tech 1 scan tool display will switch from "OFF" to "ON."

FAN 2 - When the PCM is commanding Fan 2 "ON," the Tech 1 scan tool display will switch "OFF" to "ON."

A/C CLUTCH - Tech 1 scan tool Displays "ON" or "OFF" - Represents the commanded state of the A/C clutch control relay. Clutch should be engaged when "ON" is displayed.

CALIBRATION - ID - Range 0-9999 - The PROM identification describes the particular PROM being used in the PCM. The Tech 1 scan tool display number is not the service part number.