DRIVABILITY QUICK KEY
Sensor signal ranges, fuel trim interpretation, Mode $06 walkthrough, and symptom-to-cause decoder. The scan-tool reference that gets you from PID to diagnosis fast.
Common engine sensors at three diagnostic conditions. Numbers vary by manufacturer — these are typical centerlines for naturally aspirated gas engines.
| Sensor | KOEO | Idle (warm) | 2500 RPM (no load) |
|---|---|---|---|
| MAF (g/sec) | 0 | 2–7 | 12–25 |
| MAP (kPa) | 95–102 (BARO) | 25–40 | 35–55 |
| TPS volts | 0.4–0.9 | 0.4–0.9 | 0.7–1.5 |
| ECT volts (NTC) | ~3.0 (cold) | 0.5–1.0 | 0.5–1.0 |
| IAT volts (NTC) | 2–4 | 2–3.5 | 1.5–3.5 |
| O2 upstream | ~0.45 (rest) | 0.1–0.9 oscillating | 0.1–0.9 oscillating |
| O2 downstream (post-cat) | ~0.45 | 0.6–0.8 stable | 0.6–0.8 stable |
| Knock sensor | 0 | 0–0.5 | 0–1.0 |
| Fuel pressure (psi, port-injected) | 40–60 (key on) | 35–55 | 40–60 |
| Fuel pressure (psi, GDI low side) | 50–80 | 50–80 | 50–80 |
| Fuel pressure (GDI high side · bar) | 0 | 20–60 bar | 50–150 bar |
Numbers off the chart? Don't condemn yet — first verify the reference voltage at the sensor (5V supply, good ground). Bad reference = bad signal that looks like a bad sensor.
STFT = Short Term Fuel Trim (closed-loop, real time). LTFT = Long Term Fuel Trim (learned, stored). Add them for total correction. Positive = ECU adding fuel (sees lean). Negative = pulling fuel (sees rich).
| STFT + LTFT total | Reading | Action |
|---|---|---|
| ±5% | Normal · system trimming as designed | Look elsewhere |
| ±5–10% | Drifting · monitor for trend | Document baseline |
| +10 to +25% | Lean correction · ECU adding fuel | Vacuum leak · low fuel pressure · weak injectors · contaminated MAF · false air |
| +25% (capped, MIL on) | Severe lean · system at correction limit | P0171/P0174 imminent · find leak now |
| −10 to −25% | Rich correction · ECU pulling fuel | Leaky injector · high fuel pressure · MAF over-reporting · fouled O2 |
| −25% (capped, MIL on) | Severe rich · system at correction limit | P0172/P0175 imminent · injector or fuel system |
Read STFT AND LTFT together. If LTFT is +15 and STFT is +5, ECU has learned a lean condition. If LTFT is 0 and STFT swings ±15 constantly, the system is hunting — often O2 sensor or fuel pressure regulator. Different stories, different repairs.
On V6/V8 engines, comparing fuel trims between banks isolates whether the cause is shared (intake, fuel pump) or bank-specific (injector, vacuum leak on one bank).
| Pattern | Most-likely cause |
|---|---|
| Both banks +15% | Shared: vacuum leak at intake manifold · weak fuel pump · plugged fuel filter · contaminated MAF |
| One bank +15%, other ±5% | Bank-specific: vacuum leak on the affected bank only · sticking injectors on that bank · failed pre-cat O2 |
| Both banks −15% | Shared: high fuel pressure · leaky FPR · MAF over-reporting · EVAP venting fuel vapor constantly |
| One bank −15%, other ±5% | Bank-specific: leaking injector on that bank · failed O2 sensor reading rich |
Mode $06 reports the actual test values the monitor used to decide pass/fail. The pending-fault patterns live here before a DTC sets — early warning system.
| Monitor | Key tests | What to look at |
|---|---|---|
| Catalyst | O2 switching ratio (post-cat / pre-cat amplitude) | Healthy cat suppresses switching · ratio < 0.5 = pass · approaching 1.0 = cat losing efficiency |
| O2 sensor | Response time · lean-to-rich, rich-to-lean | Should be < 100 ms · > 200 ms = lazy sensor approaching DTC |
| EVAP | Tank pressure decay, vacuum hold | Compare measured leak to threshold · approaching threshold = small leak forming (cap, hose) |
| Misfire | Per-cylinder misfire counts | One cylinder dominating = injector, plug, or compression issue on that hole |
| EGR | Flow rate at commanded position | Below threshold = restricted passages · above = stuck open |
| VVT | Cam position vs commanded | Lag > allowed = oil control valve or VVT actuator |
Mode $06 IDs are manufacturer-specific. Use the scan tool's "test ID" lookup or OEM service info to decode. Generic IDs help across brands but specifics need the make/model database.
| Symptom | Cause order (most → least likely) |
|---|---|
| Cold-start hard, runs OK warm | ECT sensor · cold-start enrichment fuel pressure · stuck-open injector · low cranking compression on one cyl |
| Surge at cruise | Lean cruise (vacuum leak · low fuel pressure) · failing MAF · TPS dirty · sticking EGR |
| Hesitation on acceleration | Weak fuel pump · weak ignition (coil, plug, wire) · dirty/restricted MAF · sticking TPS |
| Misfire at idle only | Vacuum leak · low compression on cyl · injector at low-PWM range · valve guide oil consumption |
| Misfire under load only | Weak spark (coil under load) · fuel delivery shortfall · cracked plug · weak rod bearing (rare) |
| Black smoke at start | Stuck-open injector · fuel pressure regulator failed (vacuum leak past diaphragm) · ECT shorted (cold-mode forever) |
| White smoke (steady) | Coolant in combustion — head gasket · cracked head · cracked block |
| Blue smoke | Oil consumption — valve guide seals · worn rings · turbo seals · PCV failure |
| Pinging / detonation | Carbon buildup · wrong octane · failing knock sensor (ECU not pulling timing) · lean operation · cooling system failing to control combustion temp |
A "bad sensor" reading is often a bad 5V reference, a corroded ground, or a wire chafe. Always test the reference voltage at the sensor connector before swapping the sensor. Especially on intermittent codes. A new sensor on a bad circuit comes back tomorrow.
Mode $06 holds the actual measurements that drove the DTC decision. Clearing codes also clears Mode $06 history on many vehicles. Always pull Mode $06 first — that's where you see whether a monitor is borderline or stable.
P0171/P0174 codes (system too lean) are usually NOT O2 sensor problems. They're vacuum leaks, fuel pressure, MAF contamination. The O2 sensor is doing its job — reporting a lean condition. Diagnose the cause, not the messenger.
Generic OBD-II PIDs (Mode $01) are slow and reduced-resolution. Use the OEM-specific (enhanced) mode of your scan tool for live data — faster refresh, more PIDs, better diagnosis. Generic mode is for reading codes, not for live diagnostics.
Print this. Stick it inside your toolbox lid.
FUEL TRIM QUICK
| ±5% | Normal |
| +10 to +25% | Lean correction · vacuum/fuel/MAF |
| −10 to −25% | Rich correction · injector/pressure/MAF |
| Both banks same direction | Shared cause |
| One bank only | Bank-specific cause |
KEY PIDS (IDLE, WARM)
| MAF | 2–7 g/sec |
| MAP | 25–40 kPa |
| TPS | 0.4–0.9V |
| O2 up | 0.1–0.9 osc |
| O2 dn | 0.6–0.8 stable |
DIAGNOSE ORDER
Read codes + Mode $06 → Fuel trim both banks → Compare to PID baselines → Verify sensor references → Mechanical test (vacuum, fuel pressure, compression) → Replace