GM Service Manual Online
For 1990-2009 cars only

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

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

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

Engine Electrical

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 P0562 and P0563 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

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

Theft System Learn Mode

Enabled/Disabled

Enabled

Password Learn Procedure

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