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.
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 | kPa / psi | 0-450 psi (varies with the refrigerant pressure) | PCM Controlled Air Conditioning Diagnosis |
Another TWC Test Desired | Yes/No | No | DTC P0420 TWC System 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 |
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 |
Calc. Vacuum | kPa | Varies with the engine Load | Automatic Transaxle Diagnosis |
Calc. Convertor Temp. | °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 |
Current TWC Test Complete | Yes/No | Yes | DTC P0420 TWC System Diagnosis |
Desired Idle Speed | RPM | PCM commanded idle speed (varies with calibration). | Idle Air Control System Check |
ECL | OK/Low | OK (Low with low coolant level) | Information Sensors Description |
ECT | °C-°F | 85°C-115°C / 185°F-239°F (varies with coolant temperature). | Information Sensors Description |
Engine Load | % | 11%-21% 16-29% at 2,500 RPM | Engine Load |
Engine Oil Pressure | Low/OK | OK (Low with low oil pressure) | Information Sensors Description |
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 |
FC Relay 2 | Off / On | Off (Varies with fan request) | Electric Cooling Fan Diagnosis |
Fuel Tank Pressure | mmHg / in 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 P0461 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 Trim 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 Noise Channel | Volts | 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 |
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 Counter | 0-255 Counts | 0 | DTC P0335 Diagnosis |
Med. Resolution Engine Sync. | Yes/No | Yes | DTC P0335 Diagnosis |
Number of 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 Shorted | 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 Shorted | 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 Sensor 1 | 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 |
TCC Brake Switch | Open/ Closed | Closed | Cruise Control and Transaxle |
Total Misfire Current Counter | 0-255 Counts | 0 (increase with a misfire) | DTC P0300-P0304 |
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 |
TWC Stage 1 Started | Yes/No | Yes | DTC P0420 TWC System Diagnosis |
TWC Stage 2 Started | Yes/No | Yes | DTC P0420 TWC System Diagnosis |
TWC System Related DTC is Set | Yes/No | No | DTC P0420 TWC System Diagnosis |
TWC Test Initialization Conditions Met | Yes/No | Yes | DTC P0420 TWC System Diagnosis |
TWC Test Enabled | Yes/No | No | DTC P0420 TWC System Diagnosis |
TWC Warmup Conditions Met | Yes/No | Yes | DTC P0420 TWC System Diagnosis |
Vehicle Speed | Km/h / MPH | 0 | Automatic Transaxle Diagnosis |
