For 1990-2009 cars

### On Board Diagnostic (OBD) System Check

The Powertrain OBD System Check is an organized approach to identifying a problem that is created by an electronic engine control system malfunction. The OBD system check is the starting point for any driveability complaint diagnosis. This directs the service technician to the next logical step in diagnosing the complaint. Keep the following special considerations in mind while making the OBD system check :

#### Blocking Drive Wheels

Block the vehicle drive wheels and set the emergency brake while checking the system.

#### Cold Oxygen Sensor (O2S 1)

On certain engines, the Oxygen Sensor (O2S 1) may cool down after only a short period of operation at idle. This may cause the system to go into an Open Loop operation. Run the engine at part throttle several minutes and accelerate the engine from idle to part throttle a few times in order to restore a Closed Loop operation.

Notice: Do not use a scan tool that displays faulty data. Report the scan tool problem to the manufacturer. Use of a faulty scan tool can result in misdiagnosis and unnecessary parts replacement.

Only the parameters listed below are used in this manual for diagnosis. If a scan tool reads other parameters, General Motors does not recommend these values for use in the diagnosis. For more description on the values and use of the scan tool to diagnose the PCM inputs, refer to the applicable diagnostic section. If all of the values are within the illustrated ranges, refer to Symptoms.

#### Typical Scan Tool Data Values

Scan Tool Parameter | Units Displayed | Typical Data Value | Reference
--- | --- | --- | ---
A/C High Side Pressure | Volts | 0-5.0 Volts | PCM Controlled Air Conditioning Diagnosis
A/C High Side Pressure | psi / kPa | 0-450 psi (varies with the refrigerant pressure) | PCM Controlled Air Conditioning Diagnosis
A/C Relay | Off / On | Off (On with the A/C requested) | PCM Controlled Air Conditioning Diagnosis
A/C Request | Yes / No | No (Yes with A/C ON) | PCM Controlled Air Conditioning Diagnosis
Adaptive Knock Retard | Degress | 0 | Information Sensors Description
Actual EGR Position | % | 0% | DTC P0404, P0405 and P1404 EGR Diagnosis
Air/Fuel Ratio | Ratio | 14.6:1 (varies) | Fuel Metering Modes of Operation
BARO | kPa | 65 - 104 (varies with the altitude and barometric pressure). | Information Sensors Description
Base PWM Cyl. 1-4 | mS | 1-4 varies | DTC P0200 Misfire Diagnosis
Calc. Air Flow | g/s | Varies | Freeze Frame and O2S Information
Catalyst Converter TT | °C-°F | Varies with the engine Load | DTC P0420 TWC System Diagnosis
CKP Active Counter (7X Reference) | 0-255 Counts | 0-255--Increments while cranking or running | Information Sensors Description DTC P0335 CKP Sensor Circuit Performance
CMP Active Counter | 0-255 Counts | 0-255--Increments while cranking or running | DTC P0341 and P0342 CMP Circuit Diagnosis
CMP Resync Counter | 0-255 Counts | 0 | DTC P0341 and P0342 CMP Circuit Diagnosis
Cruise | Enabled / Disable | Disable | PCM Outputs Diagnosis
Cruise | Engaged / Disengaged | Disengaged | PCM Outputs Diagnosis
Desired EGR Position | % | 0% | DTC P0404, P0405 and P1404 EGR Diagnosis
Desired Idle Speed | RPM | PCM commanded idle speed (varies with calibration). | Idle Air Control System Check
ECL | OK/Low | OK (Low with low oil pressure) | Information Sensors Description
ECT | °C-°F | 85°C - 115°C / 185°F - 239°F (varies with coolant temperature). | Information Sensors Description
EGR Delta MAP Calc. | kPa | 0 or a negative number | DTC P0401 EGR Diagnosis
EGR Decel Filter | kPa | -3 or -4 | DTC P0401 EGR Diagnosis
EGR Flow Test Count | Number of samples | 0 | DTC P0404, P0405 and P1404 EGR Diagnosis
Engine Load | % | 11%-21% 16-29% at 2,500 RPM | Engine Load
Engine Run Time | Hr: Min: Sec | Depends on time since startup. | Engine Run Time
Engine Speed | RPM | ±100 RPM from Desired Idle | Information Sensors Description Idle Air Control System Check
EVAP Canister Purge | % | 0%-100% (varies) | EVAP Control System Description
EVAP Vent Solenoid | Open / Closed | Open | EVAP Control System Description
FC Relay 1 | Off / On | Off (Varies with fan request) | Electric Cooling Fan Diagnosis
Fuel Tank Pressure | mmHg / H2O | Depends on Pressure / Vacuum | DTC P0452, P0453, P0440, P0442, P0446 and P1441 Diagnosis
Fuel Tank Pressure | Volts | Depends on Pressure / Vacuum | DTC P0452, P0453, P0440, P0442, P0446 and P1441 Diagnosis
Fuel Level | % | 0-100% Depends on fuel level | DTC P0460 Fuel Level System Diagnosis
Fuel Trim Cell | Cell number | 18-21 at idle (varies with the air flow, RPM P/N and A/C | DTC P0171 and P0172 Fuel Trim System Rich
Fuel Trin Index | % | 58-198 (varies -100% to 100%) | DTC P0171 and P0172 Fuel Trim System Rich
Generator L Terminal | Active / Inactive | Inactive | Generator Diagnosis
Generator PWM | % | 0-100% (varies) | Generator Diagnosis
HO2S 2 | mV | 0 - 1000, constantly varying | Information Sensors Description
IAC Position | Counts | 5 - 60 | Idle Air Control System Check
IAT | °C-°F | 10°-80°C (50°-176°F varies with under hood temperature) | Information Sensors Description
Ignition 1 | Volts | 12.0-15.0 Volts (Varies) | DTC P0560 System Voltage
KS Active Counter | Counts | 0-2 | Knock Sensor System Description
KS Noise Channel | Number | Increments when knock is present | Knock Sensor System Description
Knock Retard | Degrees | 0° | Knock Sensor System Description
Lean / Rich Transition | 0-255 Counts | 0 | DTC P0133 and P1133 Diagnosis
Lean / Rich Average Time | mS | 0 | DTC P0133 and P1133 Diagnosis
Long Term FT | % | 58-198 (-100% to 100% varies) | DTC P0171 and P0172 Fuel Trim System Lean
Loop Status | Open / Closed | Closed | DTC P0125 ECT Excessive Time to Closed Loop
MAP | kPa | 25 - 48 kPa | Information Sensors Description
MAP | Volts | 1-3 volts (varies with the vacuum and BARO pressure) | DTC P0106, P0107 and P0108 Diagnosis
Total Misfire Current Counter | 0-255 Counts | 0 (increase with a misfire) | DTC P0300-P0304
Misfire Current #1-#4 | 0-255 Counts | 0 (increase with a misfire) | DTC P0300-P0304
Misfire History #1-#4 | 0-255 Counts | 0 (increase with a misfire) | DTC P0300-P0304
Med. Resolution Resync | 0-255 Counts | 0 | DTC P0335 Diagnosis
Med. Resolution Engine | Yes/No | Yes | DTC P0335 Diagnosis
Mileage Since First Failure | km | Varies | Freeze Frame Information
Mileage Since Last Failure | km | Varies | Freeze Frame Information
Number of Current DTCs | Number | 0 | Freeze Frame Information
Number of catalyst Mon. Test Completed | Number of samples | Varies | DTC P0420 TWC System Diagnosis
O2S 1 | mV | 1-1000 and varying | Information Sensors
Output Driver 1 Open | 00000000 | 00000000 (A 1 appears with a short to ground or open) | PCM Outputs Diagnosis
Output Driver 1 Short | 00000000 | 00000000 (A 1 appears with a short to voltage) | PCM Outputs Diagnosis
Output Driver 2 Open | 00000000 | 00000000 (A 1 appears with a short to ground or open) | PCM Outputs Diagnosis
Output Driver 2 Short | 00000000 | 00000000 (A 1 appears with a short to voltage) | PCM Outputs Diagnosis
Power Enrichment | Active / Inactive | Inactive | Fuel Metering Modes of Operation
Purge Learned Memory | Counts | 1.00 | DTC P0440, P0442, P0446 and P1441 Diagnosis
Rich / Lean Transition | 0-255 Countes | 0 | DTC P0133 and P1133 Diagnosis
Rich / Lean Average Time | mS | 0 | DTC P0133 and P1133 Diagnosis
Rich / Lean to Lean / Rich | Ratio | Varies | DTC P0133 and P1133 Diagnosis
Short Term FT | % | 58-198 (-100% to 100% varies) | DTC P0171 Fuel Trim System Lean
Spark | Degrees | Varies | Information Sensors Description
TP Angle | % | 0% (up to 100% at wide open throttle) | DTC P0121, P0122 and P0123 TP Sensor Circuit High Voltage
TP Sensor | Volts | 0.200--0.900 volts (up to 5.0 volts at wide open throttle) | DTC P0121, P0122 and P0123 TP Sensor Circuit High Voltage Information Sensors Description
Transmission Range | Invalid/Low/Drive 2/Drive 3/Overdrive/Reverse/Park/Neutral | Park / Neutral (Auto trans only) | Information Sensors Description
TR Switch P A B C | Low / High | Low / Low / High / High | Information Sensors Description
Vehicle Speed | Km/h / MPH | 0 | Automatic Transaxle Diagnosis