GREASE-AI is only as good as the data you feed it. A 30-second log with the wrong channels will get you a generic report. A proper log with the right PIDs will get you a root cause. Here's exactly what to capture.
The #1 mistake: logging too short
Most issues don't show up at idle. Log a full cold start, a cruise at highway speed, and at least one WOT pull. Minimum 3–5 minutes of data. More is always better.
These channels are the minimum needed for GREASE-AI to generate a useful report. If any of these are missing, the analysis will be incomplete.
| Channel / PID | Why It Matters |
|---|---|
| RPM | Engine speed — baseline for everything else |
| MAP or MAF | Engine load. MAP for speed-density tunes, MAF for mass-air. Log both if available. |
| TPS (Throttle Position) | Confirms driver demand vs. actual response |
| IAT (Intake Air Temp) | Affects fueling and timing. Critical for diagnosing heat soak. |
| ECT (Engine Coolant Temp) | Required for cold-start and warm-up analysis |
| STFT Bank 1 & 2 (Short Term Fuel Trim) | Real-time fueling correction — shows lean/rich conditions immediately |
| LTFT Bank 1 & 2 (Long Term Fuel Trim) | Learned fueling correction — shows chronic lean/rich issues |
| O2 Sensor B1S1 & B2S1 (upstream) | Pre-cat oxygen sensors — validates fuel trim data |
| Ignition Timing / Spark Advance | Timing retard reveals knock, detonation, or calibration issues |
| Knock Counts / Knock Retard | Most important channel for diagnosing detonation. Log per-cylinder if available. |
| Injector Pulse Width | Shows actual fuel delivery — catches injector issues and fueling limits |
| Vehicle Speed (VSS) | Required to correlate load conditions with road speed |
| Channel / PID | Why It Matters |
|---|---|
| Battery Voltage | Low voltage causes a cascade of false codes and sensor errors |
| Barometric Pressure (BARO) | Altitude compensation — important for shops at elevation |
| EGR Position / EGR Commanded | Required for diagnosing EGR-related misfires and rough idle |
| Cam/Crank Correlation | Catches timing chain stretch and VVT issues |
| Misfire Counts (per cylinder) | Pinpoints which cylinder is misfiring — saves hours of diagnosis |
| Fuel Pressure | Rules out fuel delivery as a cause before chasing sensors |
| Boost Pressure (turbocharged engines) | Required for any forced-induction diagnosis |
| Transmission Temp | Needed for shift quality and TCC slip analysis |
| TCC Slip / Torque Converter Slip | Diagnoses converter shudder and transmission hunting |
Cold Start
Start from fully cold (overnight soak). Log the entire warm-up until ECT reaches operating temp. This catches cold-start enrichment issues, idle instability, and choke-related faults.
Idle (Warm)
5+ minutes at warm idle in park. Reveals vacuum leaks, idle air control issues, and chronic fuel trim problems that disappear under load.
Light Cruise
30–55 mph, light throttle, flat road. Best condition for catching lean conditions, EGR faults, and TCC shudder.
Highway Cruise
60–70 mph, steady throttle. Captures long-term fuel trims under load and transmission lock-up behavior.
WOT Pull
Full throttle acceleration from 30–60 mph in 3rd or 4th gear. Required for knock analysis, timing behavior, and fuel delivery limits. Do this safely on a private road.
Decel / Overrun
Lift off throttle from highway speed. Captures decel fuel cut, EVAP purge behavior, and vacuum-related issues.
Download the full PID Reference Guide
All PIDs, logging conditions, and scanner setup in a single printable PDF — take it to the shop.