One-Trip DTCs

8-02 Identify OBD II. code types.

There are two types of DTCs: one-trip or Type A faults and two-trip or Type B faults. One-trip codes are typically associated with a clear-cut or catalyst-damaging fault like an open or short in a component or circuit such as a critical engine sensor, coil, or fuel injector. During a one-trip fault the PCM immediately stores a DTC and Freeze Frame data and turns on the MIL. All comprehensive component monitor faults require only one trip to set a DTC.

A common cause of Type A DTCs are misfire events. A misfire may increase the temperature of the catalytic converter enough to damage it. OBD II rules require the MIL to blink, or fuel to be removed from the misfiring cylinder. The blinking MIL is meant to alert the driver so they can stop driving the vehicle, preventing the catalyst from damage. The blinking MIL may be intermittent based on the severity of the engine misfire. In other words, a misfire that exceeds emissions regulations may not be substantial enough to damage the catalyst, so the MIL will light on the second trip, but not blink. It is not uncommon to drive a vehicle with a misfire where the MIL switches back and forth from being on solid to blinking as the severity of the miss changes.

Two-Trip DTCs

To set a two-trip DTC (Type B), the same fault must occur during two consecutive trips to store a DTC and Freeze Frame data and illuminate the MIL. Two trip faults prevent illuminating the MIL from “false” information. The PCM stores a temporary code when it determines that a parameter is out of range for a component or system on the first trip of a two-trip emission diagnostic monitor. If the fault repeats on the next trip, the pending DTC becomes a mature (hard) code, stores Freeze Frame data, and turns on the MIL. If the fault does not occur during the next trip, the temporary DTC is cleared from the PCM memory. All system monitors are two-trip monitors.

Pending DTCs

Pending DTCs are want-to-be codes. Think of a pending code as an early warning sign that something may be wrong. A pending DTC can set for an intermittent concern; it does not indicate that a fault is present or that the fault is gone (FIGURE 8-2). A pending DTC is only an indication that a fault has recently been detected. Pending DTCs set after one trip indicating that there may be an issue that can mature and illuminate the MIL. Pending DTCs can indicate a problem that is evolving, or alert a technician to a persistent concern if it appears after a repair.

FIGURE 8-2 A pending code means that the code has not totally failed yet, but has failed once and the PCM is going to try to run the monitor again to see if it fails. If it does fail, then it becomes a hard code.

When a two-trip fault is detected for the first time, a fault-related code is stored as a pending code, but the MIL is not illuminated. Normally this is a code(s) that is waiting to be validated by a second test that has yet to run because the criteria for the test have not yet been met. The PCM is waiting for the second test to run. A pending DTC can mature into a current, or hard, DTC if the same fault repeats during the second consecutive drive cycle. The PCM will erase a pending code if the fault fails to repeat under similar driving conditions during the second consecutive drive cycle.

During diagnosis when following DTC diagnostic charts or trouble trees, the technician may be instructed to erase the DTC and then run the monitor that stored the pending DTC again by completing the required trip. If the original fault repeats, a pending code will be stored again referring you back to the diagnostic chart. If the code does not repeat normally, you will be instructed to release the vehicle back to the customer since the problem cannot be duplicated.

You can also use pending DTCs for repair verification. After performing a repair refer to the monitor status screen on your scanner that set the DTC. If the monitor has completed, check for pending DTCs for two-trip DTCs. If a pending DTC is stored, the repair did not correct the fault. If the repair is fixed, no DTC will be stored.

TECHNICIAN TIP

Some manufacturers have a “Check Mode” diagnostic function in the PCM’s software. Check Mode temporarily converts all DTCs into one-trip codes to enhance diagnosis. There are typically two methods to activate the test. The first requires the technician to select the test through the scanner manually. The second method uses a jumper wire, with the correct terminal adapter placed into a specific diagnostic connector and terminal. Refer to the manufacturer’s service information for availability and the proper procedure. After selecting the “Check Mode” function, drive the vehicle to complete the required trip (meeting all the necessary enabling criteria) and then check for any stored DTCs.

Note: The only way to tell if a pending code is present is to check for DTCs. To access pending DTCs on your scanner it is recommended to use the Generic OBD II interface. Most manufacturers do not distinguish between mature and pending DTCs.

Current DTCs

A current DTC is an indication that during the last ignition cycle a test related to the stored DTC has failed. A current DTC has failed twice if it is a Type B DTC. A Type A DTC can be current after one trip (FIGURE 8-3). Some manufacturers may supply additional code definitions such as: failed since cleared, failed this ignition cycle, not run this ignition cycle, not run since cleared, last test passed, and test ran and passed.

FIGURE 8-3 The diagram shows that the PCM erases non-Fuel System or Misfire Type B DTCs after 40 warm-up cycles. If the code set is for a Fuel System or Misfire concern, the DTC is stored for 80 warm-up cycles.

History DTCs

History DTCs are codes that were out of parameter at one time but are not currently out of range. Whenever the PCM detects a fault, it stores a DTC as a history code. To set a history DTC the failure can occur during the present trip or any previous trip but must have occurred at least once since the codes were last cleared. The PCM stores history codes in memory until a specific number of ignition cycles occur, a technician erases the DTC(s) with a scan tool, or power is disconnected from the PCM.

Permanent DTCs

Permanent DTCs are a recent addition to OBD II first seen during the model year 2010 and fully implemented by all manufacturers in 2012. The addition of permanent DTCs is designed to close the increases in readiness monitor loopholes. Permanent DTCs are not cleared in the usual way, that is, with a scan tool or by disconnecting the battery. Permanent DTCs are cleared using a unique method. “Clearing” a permanent DTC with a scan tool after a successful repair will extinguish the MIL, but it will not remove the code from the PMC memory. To clear a permanent code, the PCM must complete and pass the same test that set the DTC by performing and completing a Drive Cycle Test. Due to the severe increase in emissions permanent DTCs are stored in the PCMs non-volatile memory, which prevents erasing the code(s) by disconnecting the battery or removing power to the PCM (FIGURE 8-4).

FIGURE 8-4 Note that the Type B column on the right for a Fuel System or Misfire Code does not clear if the fault is not detected on the next consecutive trip. Misfire and Fuel System codes remain in memory and are monitored for the next 80 trips. This occurs in the Similar Conditions Window. All other Type B codes will clear if the fault does not repeat on the second consecutive trip.

Freeze Frame Data

8-03 Use Freeze Frame data to aid in engine diagnostics.

Freeze Frame data provides a snapshot of the conditions present when the DTC was stored. The PCM takes at least one single-frame snapshot of major engine operating conditions and stores them in its memory the instant the PCM sets the first DTC (FIGURE 8-5). Freeze Frame data is a standard required by OBD II that can also be used for diagnosis. The stored data is used by the PCM for comparison of similar operating conditions should the fault occur in the future. Freeze Frame data is mandated to include emission-related values from specific generic PIDs. Some manufacturers may also store non-emission-related data from other systems. Multiple frame data supplied by some manufacturers are called movies.

FIGURE 8-5 Freeze Frame data gives the technician the ability to operate the vehicle under the conditions in which the failure occurred. This also helps once the vehicle is repaired as it allows the technician to check their work.

If more than one DTC is stored, the Freeze Frame data applies to the first code set. This holds true unless a catalyst damaging Misfire or Fuel Trim code sets after the original DTC. Once the DTC and Freeze Frame data is stored it remains in the PCMs memory even if other emissions-related DTCs are set. Data is stored by priority so an exception to this rule exists. If a fault occurs that has a greater effect on emissions, such as a fuel system or misfire DTC, the PCM will overwrite any previously stored data and update the Freeze Frame data. Once the PCM stores the fuel system or misfire DTC that data can no longer be overwritten. Freeze Frame data can be lost if the battery is disconnected.

Note: It is true that on some later model vehicles the PCM may be able to retain DTCs and Freeze Frame data in its memory for several days after disconnecting power. It is still recommended to retrieve and store codes and data during the original diagnosis. If for some reason this is not done, check the PCM to see if the information is still available.

Remember that generally only one DTC will store Freeze Frame data. Newer vehicles may offer more than Freeze Frame data. Freeze Frame data provides you with the vehicle’s operating conditions when the fault occurred. The amount of information provided by Freeze Frame data varies. Early OBD II systems supply a limited number of PIDs that focus primarily on engine performance. Current model year vehicles provide increased information compared to early OBD II systems. Failure to check the vehicle under similar conditions may result in the vehicle being returned to the customer without being repaired. Record Freeze Frame data on the Repair Order (RO) before erasing any codes. Clearing a DTC also erases the stored Freeze Frame data. When multiple emission DTCs are stored in the PCMs memory, note the code that stored the Freeze Frame data.

TECHNICIAN TIP

Manufacturers store Freeze Frame data the first time a fault is detected. Freeze Frame data is not updated, or refreshed, when a test fails a second time; it is only made available when the DTC sets. The data stored in Freeze Frame is generic and therefore are actual values, not PCM-substituted values.

Freeze Frame data is found on your scan tools Generic Menu. You should always check and record Freeze Frame data after retrieving codes. A recommended procedure is to always start in the Generic side of your scan tool. Check for codes and if available access the Freeze Frame data then take a snapshot of it or record it. You can then move to checking data in either the Generic or Enhanced datastream. Following this procedure guarantees you that the operating conditions present when the DTC set are available to you in case the code(s) are accidentally erased.

TECHNICIAN TIP

Freeze Frame PIDs do not normally contain actual fault information for the DTC that set, only the operating conditions that were present when the fault occurred. Use the PID data stored in Freeze Frame information to duplicate the conditions when the fault set.

Misfire Freeze Frame Data

Some manufacturers have included additional Freeze Frame data using specific PIDs to store data in the PCMs keep alive memory (KAM) to help diagnose the root cause of misfires. This collection of PIDs is separate from the generic Freeze Frame data stored for all MIL codes and are for misfire diagnosis only (FIGURE 8-6). This data is captured at the highest misfire rate, not when the DTC is originally stored. In other words, misfire data can be stored minutes after the misfire first occurred.

FIGURE 8-6 Example of available misfire-specific Freeze Frame data from a Ford vehicle. The manufacture has decided to list out what it believes will help the technician in diagnosing a misfire on this vehicle.

Note: The availability of Misfire Freeze Frame data varies by manufacturer and if present may not be available on all scan tools since enhanced PID access varies by scan tool manufacturer. OEM scan tools will have the correct software to access this information.

Manufacturer-Enhanced Specific Freeze Frame

Manufacturers may offer additional Freeze Frame data that provides the storage of more PIDs compared to Generic Freeze Frame data. Like OBD II Freeze Frame data, manufacturer-specific Freeze Frame data is a snapshot of the vehicle’s operating condition when the DTC set. The Manufacturer Freeze Frame snapshot data details the conditions at the time when a fault occurred. Additionally, many OEMs store more than one Freeze Frame providing more historical data. Each snapshot is approximately 40 to 50 PIDs providing up to five snapshots for five different DTCs. The PCM reports the most recent conditions for each DTC and refreshes the information a maximum of once per operation cycle.

TECHNICIAN TIP

Freeze Frame and Failure Records are extremely useful diagnostic tools for you, the technician. They can be used to determine the vehicle’s operating conditions that were present when the fault occurred. Always check and record Freeze Frame or Failure Record information before clearing a code. Erasing DTCs also clears the stored Freeze Frame data.

Fail Records and DTCs

A few manufacturers use a system known as Failure Records to record data when a DTC sets. Failure Records contain similar data parameters as Freeze Frame but expand the amount of information available. To overcome the inherent limitations of Freeze Frame data, Failure Records assist technicians when multiple DTCs or a non-emission DTC sets. Additional DTC storage is typical limited to five DTCs. If there are more than five DTCs, Failure Records will not provide information for those codes.

Some vehicles will store a Failure Record any time a diagnostic test fails and DTC is set, including first-trip, Type B (two-trip) DTCs. Failure Records are refreshed or updated with current data each time a test fails. If additional DTCs are set, a Failure Record is assigned to it. When multiple DTCs set using all the available Failure Record space, the PCM replaces the oldest Failure Records with the newest DTC Failure Records.

Enhanced Trouble Code Menus

Some OEMs provide additional trouble code menus to help determine the cause of a code assisting in diagnosis. An example is General Motors. GM offers multiple layers of DTC menus that can be confusing to understand. Some of the definitions of the tests appear to duplicate each other, but there is a difference in the terminology (FIGURE 8-7). Some examples of the different menus GM offers depending on model year and vehicle include:

FIGURE 8-7 Different GM DTC tests available on a 2008 Saturn Aura. Each test provides distinct code information to help with diagnosis of any available codes.

MIL Request—Displays only codes that are currently requesting the MIL to illuminate. This means that the only codes shown are one-trip DTCs or mature two-trip DTCs. Two-trip DTCs are only listed after the second failure.

History—Also known as Continuous Codes, lists all stored trouble codes. Many technicians begin their diagnosis here, regardless of the manufacturer or terminology (FIGURE 8-8).

FIGURE 8-8 Code listed in history indicate that the fault has occurred since DTCs were cleared but may or may not be present now. Run additional DTC tests that are available to help determine if the fault is currently present or intermittent. For intermittent codes, refer only to Freeze Frame or Failure Records then test drive under similar conditions to attempt to duplicate the concern.

Last Test Fail—Displays only the most recent failure of Continuous or Main Monitor tests. Pending DTCs can also be listed here (FIGURE 8-9).

FIGURE 8-9 GM Last Test Failed DTC Test only displays the most recent Continuous or Main Monitor tests. Pending DTCs can also be listed here. Notice that the P0713 for Transmission Temperature Sensor is not shown as it is in the History menu. This verifies that Transmission Temperature Sensor fault condition is not present now.

Test Fail Since Codes Cleared—Lists all the failed tests since DTCs were last cleared.

Fail This Ignition—Only codes that have failed during the current ignition cycle. This includes Pending (first failure of a two-trip code) and Mature DTCs (FIGURE 8-10).

FIGURE 8-10 Failed this ignition codes are faults that are currently present. This can be a mature DTC (a fault that has occurred on two consecutive trips), a one-trip code, or a pending code (the first failure of a two-fault DTC).

Not Run Since Codes Cleared—For incomplete monitors. The monitors that have not run to completion will store codes here. The list can be long depending on how many monitors are not complete. If a monitor has run, as noted in Generic OBD II Monitor status by a Complete, Yes, or Done, the codes for that monitor will not be listed.

DTC Status—Lists every possible powertrain DTC. Next to the code will be a YES/NO indicating if the test has run since clearing DTCs. Codes that pass or fail will be noted. It is possible to have a DTC with both pass and fail beside it, indicating that the fault is intermittent.