11.9 Scan Data and Freeze-Frame Interpretation

Scan Data: Conditions, Units and Meaning

ASE T1 requires interpreting data stream values, freeze frame, readiness and diagnostic results. These are different records. A live PID is the controller's present interpretation, a freeze frame is a stored operating snapshot, and a monitor result belongs to a defined self-test. Before comparing them, establish when each was captured and which controller reported it.

Confirm units and naming. Pressure can be absolute or relative to atmosphere; temperature can be Celsius or Fahrenheit; an oxygen-related PID can report voltage, lambda or equivalence ratio. A familiar number with the wrong unit can reverse a diagnosis. For example, 35 kPa MAP is an absolute manifold reading, not 35 kPa of vacuum. With a stated 101.3-kPa atmospheric value, the pressure difference is 66.3 kPa. A gauge and PID should be compared on the same basis.

Building a Useful Data Capture

Choose the small group of PIDs needed to distinguish the remaining fault hypotheses. Reading many channels can slow updates and hide a brief event. A DSO may be needed when a circuit dropout is faster than scan reporting. Record engine speed, calculated load, temperature and system status with the suspect value so the capture can be interpreted in context.

Compare command with response where supported: desired and actual throttle angle, requested and actual rail pressure, or commanded and indicated cam position. Command shows what the controller requested. Feedback shows what its sensor reported. Neither alone proves physical motion or correct flow. Confirm the relevant supply, sensor and mechanism before treating the difference as a failed actuator.

Use a known physical state for rationality checks. After a sufficient soak, compare temperature PIDs with independent ambient evidence, allowing the specified tolerances and different thermal masses. At KOEO, manifold pressure should correspond to local barometric pressure on the appropriate sensor scale. A value within its electrical range can still be wrong for the actual condition. A value outside range can arise from wiring or reference loss as well as the sensor.

Interpreting Freeze Frame Without Overclaiming

Save the available freeze frame and enhanced failure records before reset. Supported fields and storage priority differ by vehicle. A snapshot may belong to a higher-priority fault rather than the first complaint the driver noticed. The time when the monitor recognized the fault need not be the instant the physical fault first occurred.

Compare the snapshot with live data under a safe reproduction of the recorded condition. A lean code set under substantial load calls for different evidence from a lean code set at idle. Record both bank corrections and whether closed-loop control was active. Do not use a fixed idle-versus-load pattern as proof of a particular component: fuel pressure, air measurement, exhaust leakage and cylinder contribution can interact.

For an example, STFT is positive 12% and LTFT positive 18% at one recorded speed/load cell. Their approximate combined correction is 30 percentage points if the tool presents additive trim values. Some strategies use multiplicative adjustments, which would instead give 1.12 times 1.18 minus 1, or 32.16%. Verify how the manufacturer and tool represent the data before calling either calculation exact. After moving to a different load cell, the learned trim can change without any new fault.

Readiness and Monitor Test Results

Readiness indicates whether a supported monitor completed its evaluation since the relevant reset. It does not alone state that the result passed. Unsupported is different from incomplete. An AIR monitor absent from the installed engine does not become a repair requirement because a generic chart lists AIR among possible monitors.

Mode $06 can show measured results and minimum or maximum limits, but the IDs, scaling and units must match the vehicle definition. A value below a maximum limit may pass, while a value below a minimum limit may fail. A raw integer is not automatically a percentage. Check whether the displayed result is current, from the last completed test or a retained history. Do not clear it and then claim the empty result proves a repair.

Reporting the Verification Result

After physical repair, repeat the diagnostic measurement that supported the fault. Then allow the affected monitor to execute under its actual enable criteria, reviewing pending and confirmed codes, result margins and any permanent record. A test that has not run is a remaining verification limit, even when the warning lamp is off.

Keep a concise capture with the operating conditions, units and interpretation. Explain whether the repair was verified at idle, under the reported load, through a commanded functional test or by a completed monitor. This prevents a successful stationary check from being mistaken for a successful highway or hot-soak test. It also gives the next technician a clear starting point if an intermittent complaint returns.

• Identify the record's source, time, units and operating state.
• Compare requested and reported values with physical evidence.
• Read monitor limits in the correct direction and scale.
• Preserve results before reset and state any unperformed verification.