OBD II GENERIC PID DIAGNOSIS

Motor, Sep 2007 by Seyfert, Karl

A wealth of diagnostic information is available on late-model OBD II-compliant vehicles, even when 'enhanced' or 'manufacturer-specific' PIDs are not accessible. It doesn't take much to use this information to its best advantage.

Some scan tools call it the global OBD II mode, while others describe it as the OBD II generic mode. The OBD II generic mode allows a technician to attach his scan tool to an OBD II-compliant vehicle and begin collecting data without entering any VIN information into the scan tool. You may need to specifically select "OBD II Generic" from the scan tool menu. Some scan tools may need a software module or personality key before they'll work in generic OBD II test mode.

The original list of generic data parameters mandated by OBD II and described in SAE J 1979 was short and designed to provide critical system data only. The useful types of data we can retrieve from OBD II generic include short-term and long-term fuel trim values, oxygen sensor voltages, engine and intake air temperatures, MAF or MAP values, rpm, calculated load, spark timing and diagnostic trouble code (DTC) count. Freeze frame data and readiness status also are available in OBD II generic mode. A generic scan tool also should be able to erase trouble codes and freeze frame data when commanded to do so.

Data coming to the scan tool through the mandated OBD II generic interface may not arrive as fast as data sent over one of the dedicated data link connector (DLC) terminals. The vehicle manufacturer has the option of using a faster data transfer speed on other DLC pins. Data on the generic interface also may not be as complete as the informaHon you'll get on many manufacturer-specific or enhanced interfaces. For example, you may see an engine coolant temperature (ECT) value in degrees on the OBD II generic parameter identification (PID) list. A manufacturer-specific data list may display ECT status in Fahrenheit or Celsius and add a separate PID for the ECT signal voltage. In spite of these and other limitations, OBD II generic mode still contains many of the trouble codes, freeze frame data and basic datastream information needed to solve many emissions-related issues.

There are nine modes of operation described in the original J1979 OBD II standard. They are:

Mode 1: Show current data

Mode 2: Show freeze frame data

Mode 3: Show stored trouble codes

Mode 4: Clear trouble codes and stored values

Mode 5: Test results, oxygen sensors

Mode 6: Test results, noncontinuously monitored

Mode 7: Show pending trouble codes

Mode 8: Special control mode

Mode 9: Request vehicle information Modes 1 and 2 are basically identical.

Mode 1 provides current information, Mode 2 a snapshot of the same data taken at the point when the last diagnostic trouble code was set. The exceptions are PID 01, which is available only in Mode 1, and PID 02, available only in Mode 2. If Mode 2 PID 02 returns zero, then there's no snapshot and all other Mode 2 data is meaningless. Vehicle manufacturers are not required to support all modes. Each manufacturer may define additional modes above Mode 9 for other information.

Most vehicles from the J1979 era supported 13 to 20 parameters. The recent phase-in of new parameters will make OBD II generic data even more valuable. The California Air Resources Board (CARB) revisions to OBD II CAN-equipped vehicles have increased the number of potential generic parameters to more than a hundred. Not all vehicles will support all PIDs, and there are many manufacturer-defined PIDs that are not included in the OBD II standard. Even so, the quality and quantity of data have increased significantly. For more information on the new PIDs that were added to 2004 and later CAN-equipped vehicles, refer to Bob Pattengale's article "Interpreting Generic Scan Data" in the March 2005 issue of MOTOR. A PDF copy of the article can be downloaded at www.motor.com.

Establish a Baseline

If you're repairing a vehicle that has stored one or more DTCs, make sure you collect the freeze frame data before erasing the stored codes. This data can be used for comparison after your repairs. The "before" freeze frame shot and its PID data establish the baseline.

As you begin your diagnosis, correct basic problems first-loose belts, weak batteries, corroded cables, low coolant levels and the like. The battery and charging system are especially important, due to their effect on vehicle electronics. A good battery, a properly functioning alterna tor and good connections at power and ground circuits are essential. You can't assume that OBD II will detect a voltage supply problem that can affect the entire system. If you have an intermittent problem that comes and goes, or random problemsthat don't follow a logical pattern, check the grounds for the PCM and any other controller in the vehicle.

If the basics check out, focus your diagnosis on critical engine parameters and sensors first. Write down what you find; there's too much information to keep it all in your head. Add any information collected from the vehicle owner regarding vehicle performance. Jot down the battery voltage and the results of any simple tests, such as fuel pressure or engine vacuum. Look at the Readiness Status display to see if there are any monitors that aren't running to completion.