Common OBD II Terms
COMMON OBD II TERMSTRIP
A trip is defined as a Key-ON, Key-OFF event in which the Powertrain Control Module (PCM) detects the following:
1. Engine coolant temperature should exceed 70°C degrees (158°F).
2. Engine coolant temperature should change more than 20°C degrees (68°F) after starting the engine.
3. Engine speed should go over 400 RPM.
TWO TRIP DETECTION LOGIC
When the PCM detects a malfunction during the 1st trip the DTC and corresponding freeze frame data are stored in the PCM's memory. The Malfunction Indicator Lamp (MIL) will not be illuminated until the fault is detected again during the 2nd trip. Certain DTC's are capable of turning the MIL light on or blinking it during the first trip.
DIAGNOSTIC TROUBLE CODES
A diagnostic trouble code (DTC) will be stored in memory after the identical malfunction has been detected consecutively on at least two separate drive cycles (not necessarily completing a Trip). A misfire detection monitor DTC can be stored immediately depending on the misfire type. A catalyst efficiency monitor DTC can be stored after three identical malfunctions are detected on three separate drive cycles. A DTC will be erased from memory after 40 engine warm-up cycles, except for misfire or fuel injection system DTCs which will be cleared after 80 warm-up cycles, if the malfunction has not been detected after the malfunction indicator lamp (MIL) is turned off. DTC memory storage requirements vary with each monitor. (Refer to OBD II Monitors for more information.)
Coding Conventions
DTC's used in OBD II vehicles consist of a letter, followed by four numbers. The letter at the beginning of the DTC identifies the function of the monitored device that has failed.
- P indicates a powertrain device.
- C indicates a chassis device,
- B is for body device.
- U indicates a network or data link code.
The FIRST number indicates if the code is GENERIC (common to all manufacturers), or if it is MANUFACTURER SPECIFIC.
- 0 indicates generic.
- 1 indicates manufacturer-specific.
The SECOND number indicates the SYSTEM that is affected, with a number between ONE and SEVEN:
1. Fuel and air metering
2. Fuel and air metering (injector circuit malfunctions only)
3. Ignition system or misfire
4. Auxiliary emission controls
5. Vehicle speed controls and idle control system
6. Computer output circuits
7. Transmission
The LAST TWO numbers of the DTC indicate the COMPONENT, or SECTION of the system where the fault is located.
ON BOARD DIAGNOSTICS II SYSTEM
Diagnostic Trouble Codes capable of illuminating the MIL when detected on the 1st trip are:
- Misfire diagnostic trouble codes
- Catalyst diagnostic trouble codes
- Closed loop control diagnostic trouble codes
FUEL TRIM
For OBD II vehicles. long term and short term fuel trim values will be shown in percentages. Freeze frame will also show fuel trim values as percentages. Fuel trim represents how much compensation the PCM must make from ideal conditions. A higher positive value for fuel trim indicates the PCM is commanding more fuel into the engine. this can be caused by vacuum leaks, restricted fuel injectors. etc. A highly negative value indicates a leaning of the engine, possibly caused by leaky injectors, etc.