11.2 Readiness, Freeze Frame and Repair Verification

Q: While diagnosing a stored P0171 (System Too Lean Bank 1) code on a 6.6L V8 medium-duty truck, a technician inspects the Mode $02 Freeze Frame data before clearing any codes. The Freeze Frame shows: Engine Speed = 3,950 RPM, Calculated Load = 86%, Vehicle Speed = 64 mph, Short-Term Fuel Trim Bank 1 = +19%, and Long-Term Fuel Trim Bank 1 = +24%. At warm curb idle (650 RPM) in the shop, Short-Term and Long-Term Fuel Trims both read within ±2%. Which fault is the most likely cause of P0171?

A restricted in-tank fuel pump module or plugged fuel filter causing fuel starvation under high load. Freeze Frame data shows that the lean condition (total fuel trim exceeding +40%) occurred exclusively at high engine speed (3,950 RPM) and heavy load (86% at 64 mph), while fuel trims at curb idle are completely normal (±2%). This pattern points to inadequate fuel volume delivery under high fuel demand—such as a weak fuel pump or restricted fuel filter—or an under-reporting MAF sensor at high airflow. A cracked intake manifold gasket or a ruptured brake booster vacuum diaphragm causes an intake vacuum leak that is worst at high manifold vacuum (curb idle) and diminishes when the throttle opens at heavy load. A stuck-open EVAP canister purge solenoid typically pulls rich fuel tank vapors into the manifold or creates an idle/low-load disturbance rather than an isolated high-load fuel starvation condition.

Q: This truck's specified EVAP trace requires a 15%–85% fuel level, an overnight soak with ECT/IAT below 122°F and within 10°F, and no inhibiting faults. Which listed conditions satisfy those stated requirements?

Fuel tank level between 15% and 85%, overnight cold soak with ECT and IAT below 122°F and within 10°F of each other, and no inhibiting DTCs. Non-continuous EVAP monitors require strict Enable Criteria before the PCM will initiate testing: fuel tank level must be between 15% and 85% (so neither liquid fuel slosh at 100% full nor excessive vapor volume below 15% distorts tank pressure decay), the engine must have undergone a cold soak with ECT and IAT below 122°F and agreeing within 10°F of each other, ambient temperature must sit within the valid window (typically 20°F–90°F), and no inhibiting DTCs (such as ECT or TP codes) can be present. Filling the tank to 100% or letting it drop to 10% suspends the EVAP monitor, an ECT of 145°F vs. 70°F IAT violates the cold-soak rationality requirement, and an active P0118 ECT code acts as a blocking fault that prevents non-continuous monitors from running.

Key Takeaways

Save freeze-frame and monitor results before a reset can remove evidence.
Continuous monitors also have enabling or suspension conditions.
A monitor pass and the required aging conditions govern permanent-code clearing.

Last updated: October 2026

Note

Readiness, Freeze Frame and Repair Verification uses stated teaching examples. Actual pin assignments, dimensions, thresholds and procedures must come from the identified engine, component and diagnostic tool.

Continuous vs. Non-Continuous Readiness Monitors, Freeze Frame (`Mode \$02`) & `Mode \$06` (Tasks H.1, H.2, H.11)

OBD-II divides system self-tests into three Continuous Monitors and the supported Non-Continuous Readiness Monitors:

Monitor Classification	Specific OBD-II Monitors Included	Execution Frequency & Diagnostic Function
Continuous Monitors (3 Total)	1. Misfire Monitor (`CKP` acceleration variance); 2. Fuel System Monitor (`STFT`/`LTFT` authority limits); 3. Comprehensive Component Monitor (`CCM`)	Run repeatedly when the relevant enabling conditions are met; rough-road or other conditions can suspend a monitor. The `CCM` checks electrical inputs and outputs for opens, shorts to ground, shorts to voltage, and basic rationality.
Non-Continuous Monitors (Gasoline Spark-Ignition)	1. Catalyst Efficiency; 2. Heated Catalyst; 3. Evaporative (`EVAP`) System; 4. Secondary Air Injection (`AIR`); 5. Oxygen (`O2` / `A/F`) Sensor; 6. Oxygen Sensor Heater; 7. `EGR` and/or `VVT` System	Run at their calibrated frequency (or during an engine-off soak) only after strict Enable Criteria are satisfied. Display status as `READY / COMPLETE` or `NOT READY / INCOMPLETE` (or `UNSUPPORTED`).

Interpreting Freeze Frame (`Mode \$02`) and `Mode \$06` Test Data

Freeze Frame (Mode \$02): Captures a single-moment snapshot of engine PIDs at the recorded fault event under the implementation's storage rules: Engine RPM, Calculated Load %, ECT, IAT, MAP / MAF, STFT and LTFT (Bank 1 & Bank 2), Vehicle Speed (VSS), Fuel Rail Pressure (FRP), and Fuel System Status (Closed Loop vs. Open Loop). This tells the technician how to duplicate the complaint: for example, if P0171 (System Too Lean Bank 1) set at 680 RPM, 18% Load, 0 mph, and +28% LTFT, the fault is an intake manifold vacuum leak (which affects idle most severely). Conversely, if P0171 set at 3,800 RPM, 88% Load, 62 mph, and +26% LTFT while idle fuel trims are near 0%, the fault is fuel delivery starvation under high fuel demand (weak fuel pump, plugged fuel filter, or under-reporting MAF sensor).
- Freeze Frame Overwrite Priority: Standard Mode \$02 memory holds one Freeze Frame snapshot. A Type A misfire or fuel system DTC automatically overwrites an existing lower-priority Type B Freeze Frame record.
Mode \$06 (On-Board Monitoring Test Results): Displays the raw engineering test values and minimum/maximum pass/fail limits that each monitor measured during its latest evaluation. Technicians use Mode \$06 (Monitor ID \$A1–\$AC for Misfire Data) to identify which specific cylinder is intermittently misfiring (comparing Exponentially Weighted Moving Average [EWMA] 10-trip misfire counts and last-driving-cycle misfire counts) even when the misfire rate has not yet crossed the Mode \$03 DTC threshold, or to see how close a catalytic converter's oxygen storage index is to failing after a repair.

Performing an OBD-II Drive Cycle to Verify Repairs and Reset Monitors (Task H.11)

After repairing an emissions or driveability fault, you cannot simply start the truck in the service bay, observe that the MIL is off, and release the vehicle. If the repaired monitor is still INCOMPLETE, the PCM has not yet tested the circuit—and any Permanent DTC (Mode \$0A) will remain stored.

Step-by-Step Drive Cycle Verification Procedure

Execute the Actual OEM Drive Trace: Obtain the trace for the engine and monitor being verified. Soak, speed, load and deceleration intervals vary. Use a safe route or permitted test facility; obey traffic conditions and never omit necessary braking to complete a monitor. Some EVAP tests run after shutdown, so a warm shop idle or a generic highway trip may not exercise them.

Tip

Worked Diagnostic Scenario — Why a Replaced EVAP Purge Valve Failed to Clear P0455 (Mode \$0A): A fleet technician replaces a stuck-open EVAP canister purge valve on a 2019 Class 5 gasoline service truck with stored P0455 (EVAP System Large Leak Detected), clears the codes with a scan tool, and road-tests the truck for 20 minutes. Upon return, Mode \$03 is empty and the MIL is off, but the state inspection computer rejects the truck because P0455 is still present as a Permanent DTC (Mode \$0A) and the EVAP monitor reads INCOMPLETE. Checking live PIDs reveals Fuel Level = 96% (the driver filled the tank to the brim before bringing the truck to the shop) and start-up ECT was 188°F. Because the EVAP monitor requires a cold soak (ECT < 122°F within 10°F of IAT) and a fuel level between 15% and 85%, the monitor was blocked from running! Draining/consuming fuel down to 60%, allowing an overnight cold soak, and executing the OEM cold-start drive trace allows the EVAP monitor to run, transition to COMPLETE, and satisfy the required monitor passes and aging conditions before the PCM can erase the permanent code.

Source Check — October 13, 2026

California BAR distinguishes readiness, permanent codes and vehicle-specific inspection exceptions; inspection acceptance depends on the actual jurisdiction. Manufacturer or regulator reference.

A Complete Repair-Verification Record

Start with the actual fault and the tests that originally supported it. Repeat the physical check after repair: pressure, flow, voltage drop, waveform or leakage as appropriate. Then review current and pending codes and the supported monitor status. Unsupported means the vehicle does not report that monitor; incomplete means a supported test has not completed. A complete monitor can record a failure, so readiness alone is not a passing result.

Use the published enable criteria for the particular monitor. Record fuel level, startup temperature, ambient conditions and any inhibiting faults that prevented execution. A cold-start monitor may need a soak, while an engine-off test may need the specified preceding drive and time after shutdown. Do not promise that one generic drive cycle clears every permanent code. Several passing evaluations or additional aging conditions can be required.

Distinguish technical verification from local inspection acceptance. The inspection program may use its own supported-monitor allowances, permanent-code rules and vehicle exceptions. Document the monitor result and remaining limitations accurately. If a permanent code persists after the repair is physically confirmed, consult the OEM clearing logic and applicable inspection guidance instead of replacing another part solely to remove the record.

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OBD-II Fault Maturation, Readiness Monitor Execution & Permanent DTC Clearing Flow

Test Your Knowledge

While diagnosing a stored P0171 (System Too Lean Bank 1) code on a 6.6L V8 medium-duty truck, a technician inspects the Mode $02 Freeze Frame data before clearing any codes. The Freeze Frame shows: Engine Speed = 3,950 RPM, Calculated Load = 86%, Vehicle Speed = 64 mph, Short-Term Fuel Trim Bank 1 = +19%, and Long-Term Fuel Trim Bank 1 = +24%. At warm curb idle (650 RPM) in the shop, Short-Term and Long-Term Fuel Trims both read within ±2%. Which fault is the most likely cause of P0171?

A cracked intake manifold runner gasket leaking unmeasured air downstream of the throttle body

A ruptured brake booster vacuum diaphragm allowing excess air into the intake plenum

A restricted in-tank fuel pump module or plugged fuel filter causing fuel starvation under high load

A stuck-open evaporative (EVAP) canister purge solenoid drawing fuel vapors into the intake

Test Your Knowledge

This truck's specified EVAP trace requires a 15%–85% fuel level, an overnight soak with ECT/IAT below 122°F and within 10°F, and no inhibiting faults. Which listed conditions satisfy those stated requirements?

Fuel tank filled to 100% capacity, engine warmed up to 195°F, and all Confirmed DTCs cleared

Fuel tank level at 10%, cold engine coolant temperature at 145°F, and intake air temperature at 70°F

Fuel tank level at 50%, an active P0118 Engine Coolant Temperature Circuit High code stored, and ambient temperature at 65°F

Fuel tank level between 15% and 85%, overnight cold soak with ECT and IAT below 122°F and within 10°F of each other, and no inhibiting DTCs

Sections you finish are checked off in the contents.

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11.3 Service Literature and Diagnosis with DTCs

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ASE T1 Gasoline Engines (Medium/Heavy Truck)