Car Diagnostics
OBD-II scanner

12 OBD PIDs worth logging every day — early problem detection without Check Engine

The most useful OBD-II PIDs for daily monitoring: LTFT, coolant temp, MAF, boost pressure, misfire counter. How to set up automatic logging in Car Diagnostics and what to do with the trends.

AI-generated 3 min read

Check Engine is “the last warning”. Before that, the ECU accumulates a bunch of parameters that degrade for weeks. If you log them daily — you can spot a problem 2-4 weeks before the light. Below: 12 PIDs I’d prioritise, and how to set up automatic logging in Car Diagnostics.

What “daily log” means

A short (30-60 sec) snapshot of key parameters at operating temperature and normal load. Taken, say, during your daily commute. Over 30 days you have 30 snapshots showing the trend. Trend beats a single reading.

12 PIDs to log

Engine — fuel side (4)

  1. RPM (Engine RPM) — baseline, needed to normalise everything else
  2. LNGFT1 (LTFT bank 1) — see Fuel Trim post. Norm ±5%, trending toward +10% over 2 weeks = signal.
  3. SHRTFT1 (STFT bank 1) — ±5% oscillations at warm idle. Wider oscillations = O2 sensor problem.
  4. MAF (Mass Air Flow) — expected at idle: 0.8 × displacement_L g/s. So for 2.0 L → ~1.6 g/s. Under-reported by 20%+ → MAF fouled.

Engine — temperatures (3)

  1. ECT (Engine Coolant Temp) — stable 185-205°F (85-95°C) in motion. Trending downward (150-165°F) = thermostat dying (see winter codes).
  2. IAT (Intake Air Temp) — should be within ambient +5 to +30°C. Under-reading → thermostat open. Over-reading → poor under-hood ventilation.
  3. OilTemp (where supported) — 195-230°F in motion; > 265°F — critical, oil cooling isn’t keeping up.

Engine — load (2)

  1. LOAD_PCT (Calculated Engine Load) — 20-30% at idle, 50-70% on highway. Sharp jumps at constant speed → sensor problem.
  2. TIMING_ADV (Timing Advance) — stable in warm operation; oscillations = knock sensor or fuel problem.

O2 sensors (2)

  1. O2_B1S1 (Upstream O2 voltage) — should oscillate 0.1-0.9V at 1-2 Hz. Smooth or slowed oscillations = tired sensor.
  2. O2_B1S2 (Downstream O2 voltage) — should sit stable at 0.6-0.8V. If it also oscillates like S1 → catalyst not holding, P0420 coming.

Special (1)

  1. MIL_MILES (Distance since MIL cleared) + WARM_UPS_SINCE_CODES_CLEARED — if you just cleared codes, how many miles passed. Important for post-repair validation.

How to log

Car Diagnostics has Automatic Session Log — on every engine start it silently logs the listed PIDs (does not drain the phone battery). Data syncs to the cloud on Wi-Fi.

Setup:

  1. Menu → Settings → Auto-log
  2. Pick the 12 PIDs above (available on paid tier)
  3. Frequency: 1 Hz (once per second) — optimal for trends; for deeper analysis — 5-10 Hz

Trend analysis — an example

Plot LTFT over 30 days:

  • Day 1-14: LTFT +3% at idle (normal)
  • Day 15-21: LTFT +8% (drift starts)
  • Day 22-30: LTFT +14% (almost at P0171 threshold)

That’s the classic slow MAF degradation — sensor covered in oil film from the PCV system. No code yet, but it needs cleaning (see the P0300 post for methodology).

Our Prediction Agent does this automatically — sends a weekly short report: “MAF needs cleaning in 3-6k mi”, “Catalyst bank 1 — 3-6 months to P0420”, etc.

What NOT to log daily

  • Freeze Frame — captured once when a code appears
  • Mode 06 — monthly is enough
  • Battery SOH (for EV) — monthly-bimonthly
  • VIN, Calibration ID — once at first connect

Auto-logging these is pointless and drains battery.

Try it on the free tier — 3 PIDs simultaneously in Live Monitor. Automatic log of 12+ PIDs — on paid tier with cloud sync.