SCOPE PATTERN REFERENCE
Idealized waveforms for the patterns you'll see most: ignition, O2, MAF, TPS sweep, crank/cam. Each shows good, marginal, and failed signatures with what's wrong and what to check.
SCOPE 01 · IGNITION SECONDARY (KV PATTERN)
| State | What you see | Most-likely cause |
|---|---|---|
| Good | Firing line 6–12 kV · spark line 1–2 ms duration · 3+ oscillations after | Healthy plug, coil, wire |
| Marginal | Firing line 15–25 kV · spark line shorter · oscillations damped | Worn plug gap · plug fouling · weak coil · high-resistance plug wire |
| Failed | Vertical spike with no horizontal spark line | Open secondary — failed coil, broken wire, fouled plug shorted to ground · misfire DTC for this cyl |
SCOPE 02 · UPSTREAM O2 SENSOR
| State | What you see | Most-likely cause |
|---|---|---|
| Good | Oscillates 0.1–0.9V, > 1 cross per second at idle, fast transitions (< 100 ms) | Healthy sensor, fuel control loop tight |
| Lazy | Slow rise/fall · narrow oscillation range (0.3–0.7) · < 0.5 cross/sec | Sensor aging · silicone contamination · sub-spec heater |
| Stuck | Flat at 0.45V · no oscillation | Dead sensor · heater open · 5V reference fault · disconnected ground |
SCOPE 03 · MAF · SNAP-THROTTLE TEST
| State | What you see | Most-likely cause |
|---|---|---|
| Good | Smooth ramp to 4.0–4.8V at WOT · fast return to baseline | Sensor healthy, signal clean |
| Noisy | Erratic spikes and dropouts during sweep | Contaminated hot-wire (oil from K&N air filter) · breaking solder joint · split intake boot pulling false air |
| Dead | Signal stays at idle level or 0V regardless of throttle | Failed sensor · wiring open · 5V reference fault · ECU input damaged |
SCOPE 04 · TPS SWEEP
| State | What you see | Most-likely cause |
|---|---|---|
| Good | Smooth linear ramp idle (0.4–0.9V) to WOT (4.0–4.8V) over slow sweep | Healthy potentiometer · clean wiper contact |
| Dead spots | Steps or pauses in the ramp · short flat sections | Worn carbon track at common throttle position · ECU sees momentary fixed values |
| Dropout | Signal drops to 0V briefly mid-sweep | Open in track or wiper · drivability complaint of momentary stall/surge during accel |
SCOPE 05 · CRANK POSITION (HALL EFFECT)
| State | What you see | Most-likely cause |
|---|---|---|
| Good | Clean square wave · 0V to 5V transitions · sync gap visible (1–2 missing teeth) | Sensor healthy · proper air gap · clean tone wheel |
| Dropout | One or more short or missing pulses · signal returns then drops again | Damaged tone wheel tooth · debris on tone wheel · degraded sensor · marginal air gap |
| Dead | Flat line · no signal during crank | Failed sensor · open wiring · 5V/ground reference fault · sheared key on tone wheel |
VR (variable reluctance) sensors produce sine wave (not square wave) at low RPM. Cranking VR signal should be at least 0.5V peak-to-peak. Below that, ECU can't decode and no-start results.
A "flat" O2 sensor at 5V/div looks fine but at 1V/div you can see the slow oscillation that indicates a lazy sensor. Always scale appropriately. Time base too long hides individual events; too short misses the pattern. Adjust scales until the pattern fills the screen.
Most scope signal problems are bad grounds. Check ground voltage drop from the sensor case (or ground pin) back to the battery negative under load. Should be < 100 mV. Higher than that means resistance in the ground path that distorts every signal on that circuit.
Cranking or idle KV patterns don't show what happens under load. Snap-throttle or driving-test the engine while scoping ignition secondary. Plugs and coils often look fine at idle and fail when actually called on under boost or WOT.
A weird waveform on a 5V-reference sensor (TPS, MAF, MAP) could be the sensor — or the 5V reference is at 4.2V because the ECU pin is dragging it down. Always check reference voltage at the sensor connector before condemning the sensor.
Print this. Stick it inside your toolbox lid.
| Signal | Good | Failed signature |
|---|---|---|
| Ignition 2nd | 6–12 kV firing · 1–2 ms spark | Vertical spike, no spark line = open |
| O2 upstream | 0.1–0.9V oscillation · >1 cross/sec | Flat 0.45V · stuck · lazy |
| MAF snap | Smooth ramp to 4.0–4.8V | Noisy spikes · contamination |
| TPS sweep | Linear 0.5V → 4.5V smooth | Dead spots · dropout to 0V |
| CKP square | Clean 0–5V edges · sync gap | Missing pulses · dead |
Always verify reference voltage + ground before condemning a sensor. Scale to fit the pattern.