This is the easiest lab in the manual: 80% of graded attempts pass (2,116 of 2,634 in our platform data). Use it to get every student one green run in the first session — that early win matters more than the content.

The most common failure is a misspelled or unassigned tag: the grader reports "LIGHT = false, expected true" because the coil is driving a tag the test never reads. Teach students to check tag names against the I/O table before debugging logic.

Pass rate drops to 51% here (444 of 868 graded attempts) — the first real filter. The usual mistake is placing the contacts on a parallel branch (OR) instead of in series; the "Only SWITCH_1 on → LIGHT off" case exists precisely to catch it.

Frame it industrially before students touch the editor: a motor that starts only when the guard is closed AND the start button is pressed. Students who anchor series = "all conditions required" stop guessing.

Pass rate 51% (259 of 503 graded attempts) — nearly identical to the AND lab, and the mirror-image mistake: contacts in series when a branch was needed. The two single-switch cases fail together when that happens, which makes the diagnosis easy to teach.

A good five-minute extension for fast finishers: combine Labs 2 and 3 into (SWITCH_1 OR SWITCH_2) AND a third condition on paper, and ask where the branch has to close.

Pass rate 53% (194 of 368 graded attempts). The classic error is using an NO contact — the lamp then does the exact opposite of the brief, and the "STOP released → LIGHT on" case fails.

This is the right moment for the examine-if-closed conversation: the contact symbol describes the logic condition, not the physical button. Students who conflate the two will struggle again in Lab 7 — invest the ten minutes here.

Pass rate 61% (210 of 345 graded attempts). Students who overthink it add a latch they have not been taught yet — and then fail the "Release START → LIGHT off" case. That failure is a teaching gift: the grader is checking the naive circuit on purpose.

Close the session with the cliffhanger, not the answer: "next week we make it stay on." The discovery framing measurably improves engagement with Lab 6.

Pass rate 69% (218 of 318 graded attempts) — the highest of the latching labs, because it is one rung with one new element. Let it be a confidence builder between the Lab 5 discovery and the Lab 7 filter.

The "Idle → LIGHT off" case catches students who drive the coil unconditionally. It looks trivial; it is the first time the grader checks what the program does when the operator does nothing.

Expect a wall: only 23% of graded attempts pass (148 of 631) — the second-hardest lab in this manual. The dominant error is putting an NC contact on the reset rung, carried over from Lab 5. An NC /STOP is true every scan the button is NOT pressed, so it resets the latch continuously and the lamp never latches.

That mistake surfaces as "LIGHT = false, expected true" — the single most common failure message across our whole platform (2,171 of 3,715 failed attempts end that way). Put the NO-vs-NC-on-reset question on the board before students run anything.

Budget a full session and expect multiple graded attempts per student; in our data most students who eventually pass a scenario need more than one run at this stage of the course.

Pass rate 25% (98 of 397 graded attempts). Two mistakes dominate: writing PT := 2 (seconds) instead of 2000 (milliseconds), and wiring LIGHT straight from SWITCH so it ignores the timer entirely — the 500 ms case exists to kill that shortcut.

This is also where students first see a program that is not a pure rung: declaration, call, then a rung reading .Q. Walk through why the call happens every scan — it prevents a whole family of "my timer never runs" questions in Labs 9–11.

Pass rate 43% (96 of 222 graded attempts) — better than Labs 7 and 8 individually, because students who survived those two have the pieces; the work here is composition. The common failure is omitting the reset rung, which fails only the 3.5 s case.

Ask students to predict the timing diagram on paper before running. The mid-cycle test case (motor still on at 1.5 s) rewards the ones who actually reasoned about time rather than toggling until green.

Pass rate 28% (52 of 189 graded attempts). The subtle failure is the restart gate: without /BOX_CNT.Q in the start rung, a full carton can be restarted without resetting — only the fourth test case catches it, so students see three greens and one stubborn red. That is a realistic commissioning experience; say so.

CU is edge-triggered: students who hold BOX_EYE closed and expect the count to climb get exactly one count. It is worth demonstrating live before they ask.

This is the hardest graded lab in the manual: an 18.7% pass rate (127 of 679 graded attempts) in our platform data. Treat it as the practical exam and schedule a double period — students who pass it cold have genuinely learned the material.

The concept that separates passers from non-passers is fault-latch semantics: releasing an E-stop must NOT restart anything or clear the fault. Students consistently reset the fault from the E-stop input itself; the third test case fails them for it, exactly as a safety auditor would.

Partial credit is easy to award by hand if your course needs it: the five test cases are independently reported, and the first two (start/stop and seal-in) map to the traditional 3-wire circuit taught with hardware trainers.

Pass rate 45% (175 of 387 graded attempts) — higher than the motor lab because students attempt it after everything else, and many iterate over several sessions. Grading takes ~8–9 seconds per run (the grader simulates full light cycles in accelerated time), so warn students the spinner is normal.

The "Never both green simultaneously" case is the one to talk about afterwards: it is a property test, not a step test. It fails programs that happen to cycle correctly but have no structural guarantee against conflicting greens — the difference between code that works and code that is safe.

Struggling students should be routed to the staged traffic-light scenarios (also free) and can return to the capstone after stage 4; strong students can be asked to add a pedestrian-crossing phase on paper as an extension exercise.