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3.5 Scan Tool Diagnostics

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ASE Advanced Engine Performance Specialist L1

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1 Chapter 1: Introduction & Exam Overview

1.1 Current Exam Facts 1.2 How to Use This Guide

2 Chapter 2: General Powertrain Diagnosis

2.1 Engine Mechanical Diagnosis 2.2 Fuel System Pressure and Flow 2.3 Charging and Starting System Diagnosis 2.4 Drivetrain Issues Affecting Engine Performance 2.5 Diagnostic Strategy and Logical Sequencing

3 Chapter 3: Computerized Powertrain Controls

3.1 OBD-II Standards and Communication 3.2 Input Sensors 3.3 Output Actuators and ECM Control 3.4 Fuel Trim and Oxygen Sensors 3.5 Scan Tool Diagnostics 3.6 Network Communication and U-Codes

4 Chapter 4: Ignition Systems

4.1 Ignition System Types and Coil Operation 4.2 Primary Ignition Diagnosis 4.3 Secondary Ignition Diagnosis 4.4 Spark Plugs and Diagnostic Reading 4.5 Ignition Timing and Knock Sensor

5 Chapter 5: Fuel and Air Induction

5.1 Air Induction System Diagnosis 5.2 Port Fuel Injection (PFI) 5.3 Gasoline Direct Injection (GDI) 5.4 Turbocharger and Supercharger Diagnosis 5.5 Variable Valve Timing and Intake Tuning

6 Chapter 6: Emission Controls

6.1 Emission Standards and Pollutants 6.2 Catalytic Converter Operation and Diagnosis 6.3 EGR System Diagnosis 6.4 EVAP and PCV Systems 6.5 Air Injection and Secondary Air

7 Chapter 7: I/M Failure Diagnosis

7.1 Inspection and Maintenance Programs 7.2 Drive Cycle for Readiness Monitors 7.3 Diagnosing P0420/P0430 Catalyst Codes 7.4 Diagnosing Misfire and Multiple-Cylinder Misfire Codes 7.5 Diagnosing Fuel Trim and Lean/Rich Codes

3.5 Scan Tool Diagnostics

Key Takeaways Explain This Topic AI

Modes $01 (live data), $02 (freeze frame), $03 (stored DTCs), $06 (non-continuous monitor results), $07 (pending DTCs), and $0A (permanent DTCs) are the most useful services on a modern OBD-II vehicle.

A code in mode $07 (pending) means the fault has been seen once but has not matured to a confirmed DTC; a code in mode $0A (permanent) cannot be cleared by mode $04 and only erases after the ECM verifies the fault is gone.

Generic OBD-II shows only emissions-related data; Enhanced or OEM-mode access exposes transmission, ABS, body, and proprietary calibration data needed for non-emissions diagnosis.

Mode $06 displays each non-continuous monitor's MIN threshold, MAX limit, and CURRENT value; a CURRENT outside MIN/MAX is a failed monitor even if no MIL is on yet.

Bidirectional controls let the technician command actuators (injector kill, coil kill, EVAP purge, EGR position, electronic throttle sweep) and confirm that a circuit and its ECM driver work.

Last updated: May 2026

Scan Tool Diagnostics

A professional scan tool exposes two parallel data worlds. Generic OBD-II is the standardized SAE J1979 view every tool can read on every 1996-and-newer vehicle. Enhanced / OEM access uses manufacturer-specific protocols on top of CAN to expose transmission, ABS, body, and calibration data not available in generic mode. A safe L1 habit is to start in generic, read modes $03, $07, $0A, $02, and $06 before clearing anything, then switch to enhanced for the specific modules involved.

The Ten Diagnostic Services

  • Mode Name Typical Use
  • $01 Show current data Live PIDs: RPM, MAF, fuel trims, O2 voltages, IAT, ECT, MAP, timing advance

$02 Show freeze frame data A frozen snapshot of all PIDs at the moment the most recent confirmed DTC set — invaluable for intermittents

$03 Show stored confirmed DTCs The codes that turn on the MIL

$04 Clear DTCs and reset monitors Erases stored, pending, and freeze frame, and resets every readiness monitor to not-ready

$05 O2 sensor test results Legacy on CAN vehicles; data is now reported through $06

$06 On-board monitor test results Detailed pass/fail data for catalyst, EVAP, EGR, O2, secondary air, and misfire monitors

  • $07 Show pending DTCs One drive cycle since the fault was seen; one more matures it to $03
  • $08 Request control of on-board system (bidirectional) Commands actuators on or off
  • $09 Vehicle information VIN, calibration ID, calibration verification number (CVN), in-use performance ratios
  • $0A Show permanent DTCs Cannot be cleared by $04; ECM clears them only after a passed drive cycle
  • Why Permanent Codes Exist

Mode $0A was added to OBD-II in 2010 specifically to defeat the clear-and-go trick used to pass state emissions inspections. A vehicle with a permanent code present cannot pass an OBD-II tailpipe waiver — even if the MIL is off — until the ECM itself decides the fault has been fixed and erases the permanent code over a complete drive cycle.

Generic vs Enhanced

  • Generic OBD-II Enhanced / OEM
  • Modules visible ECM/PCM only Every module on the network
  • DTC range Emissions-related P, C, B, U Full DTC range including "$U" subcodes
  • Live data SAE J1979 standard PIDs Hundreds of proprietary PIDs
  • Bidirectional Limited (mode $08) Full actuator and calibration access

A P0420 can be diagnosed with generic data alone. A transmission shift-quality complaint usually cannot — that requires OEM access to TCM data.

Mode $06 Monitor Data

Non-continuous monitors (catalyst, EVAP, EGR, secondary air, O2, O2 heater) report numerical results in mode $06. Each test reports:

MIN — the lower acceptable threshold.

MAX — the upper acceptable limit.

CURRENT — the latest measured value.

If CURRENT is inside MIN-to-MAX, the monitor passed. Outside that window, the monitor failed. A monitor can fail in $06 before a confirmed DTC sets in $03, giving the technician an early warning of trending problems (e.g., a catalyst whose efficiency margin is shrinking each drive cycle).

Bidirectional Controls (Mode $08 and Enhanced)

Bidirectional commands let the scan tool order the ECM to operate an actuator. Common examples:

Output state test — Cycle each ECM driver on and off so the technician can confirm relays, cooling fans, EVAP solenoids, EGR valves, and warning lamps.

Injector kill / coil kill — Disable one cylinder at a time and watch RPM drop; the cylinder with the smallest RPM drop is the weak one.

IAC count adjustment — Force the bypass open or closed to confirm idle response on cable-throttle systems.

Electronic throttle sweep — Command the throttle plate from 0 to 100 percent while watching both TPS signals on the same scan-tool graph.

EVAP service-bay test — Pressurize or vacuum the EVAP system and let the ECM monitor leak-down.

VVT solenoid command — Step the cam advance and compare commanded vs actual.

What Bidirectional Tells You

If a bidirectional command turns the actuator on and the technician hears or sees it work, the circuit and the ECM driver are good — the next time the actuator fails to operate during normal driving, the cause is the control strategy , the input signal that should be commanding it , or an intermittent connection , not the actuator itself.

If the bidirectional command does not operate a known-good actuator, the failure is in the ECM-to-actuator wiring or the ECM driver. Replacing the actuator at that point is a parts-changer move.