C-42 Sanitation System exam questions, with answers

10 practice questions for the CSLB Sanitation System trade exam, each with the answer, a plain explanation and the source it rests on.

From our free C-42 test · Updated 14 October 2026

These are not real exam questions. CSLB does not release its test questions or answer keys, and nobody else has them. We write our own questions from the C-42 outline and the codes, standards and references CSLB lists for the exam. Each answer names its source so you can check it.

The C-42 exam has 5 sections. The questions below are grouped by section, with the share of the exam each one carries. Read the question, pick an answer, then open “Show the answer”. The other questions of the free test, scored section by section, are in the free 25-question C-42 test.

Planning and Estimating · 27% of the exam

Planning sanitation systems; estimating sanitation systems.

1. A proposed leachfield site has a seasonal vernal pool near the back property line. Under the State Water Board OWTS Policy (Tier 1), what is the minimum horizontal setback from the dispersal system to the vernal pool?

  1. 50 ft
  2. 100 ft
  3. 150 ft
  4. 200 ft
Show the answer

Answer: D. Section 7.5.5 of the State Water Board OWTS Policy (Tier 1) sets 200 ft from vernal pools, wetlands, lakes, ponds and other surface water bodies. 100 ft is the Tier 1 setback for springs, flowing streams and private wells, 150 ft is for a public water well when the dispersal system is 10 ft deep or less, and 50 ft is the 2025 CPC clearance from a building sewer to a stream.

Source: OWTS Policy, Tier 1, section 7.5.5 (with 7.5.2, 7.5.4, 7.5.6)

2. You are pricing the hole for a new precast septic tank. Measured as a neat-line box, the excavation is 12 ft long, 8 ft wide and 7 ft deep; the tank itself is 6 ft tall. Using 27 cu ft per cu yd, how many cubic yards of soil must be dug, to the nearest 0.1?

  1. 21.3 cu yd
  2. 24.9 cu yd
  3. 74.7 cu yd
  4. 89.8 cu yd
Show the answer

Answer: B. The hole is 12 x 8 x 7 = 672 cu ft, and 672 / 27 = 24.9 cu yd (27 cu ft make 1 cu yd, NIST Handbook 44). 21.3 uses the 6-ft tank height instead of the 7-ft hole depth, 74.7 divides by 9 (square feet per square yard) and 89.8 divides by 7.48, the gallons in a cubic foot.

Source: NIST Handbook 44, Appendix C, units of volume (27 cubic feet = 1 cubic yard)

New Septic System Installation · 26% of the exam

Installing septic tanks; installing leachlines and seepage pits; testing and grading over septic systems; installing alternative treatment systems; specialty sanitation systems.

3. Your crew is setting an IAPMO-approved polyethylene septic tank on a new system under the State Water Board OWTS Policy (Tier 1). The foreman wants to bed and backfill it the way the crew does concrete tanks. Under section 8.2.3, what must govern how this tank is installed?

  1. The general contractor's site grading plan
  2. The crew's usual method for concrete tanks
  3. The building inspector's standard tank detail sheet
  4. The tank manufacturer's installation instructions
Show the answer

Answer: D. Section 8.2.3 of the State Water Board OWTS Policy (Tier 1) limits new and replacement septic tanks to IAPMO-approved tanks or tanks certified by a California registered civil engineer, and says their installation shall be according to the manufacturer's instructions. Plastic tanks have their own bedding, backfill and burial limits, so concrete-tank habits, a grading plan or a generic detail do not replace the maker's instructions.

Source: OWTS Policy, Tier 1, section 8.2.3, State Water Board OWTS Policy (Water Quality Control Policy for Siting, Design, Operation, and Maintenance of OWTS), amended 2023

4. The leach trenches are dug and open, and the rock truck driver wants to drive across the trench area to reach the far end of the field. What does EPA guidance (the EPA Onsite Wastewater Treatment Systems Manual, national guidance) say?

  1. Drive across only while the soil is dry and firm enough
  2. Keep equipment off the field so the soil is not compacted
  3. Drive across only after the rock is in all the trenches
  4. Drive across on plywood sheets to spread the tire load
Show the answer

Answer: B. The EPA manual says compaction and smearing of the infiltrative surface come from equipment traffic and vibration, and that equipment should be kept off the infiltration field. Waiting for the rock, dry soil or plywood still puts truck loads on the soil the field depends on.

Source: EPA Onsite Wastewater Treatment Systems Manual, section 4.4 (SWIS construction)

Sewer Lateral Installation and Repair · 10% of the exam

Installing sewer laterals; maintaining and repairing existing sewer systems.

5. Your crew is laying a straight 4-in building sewer 80 ft long from a house to the public sewer connection, and the AHJ has not approved any reduced slope. Under 2025 CPC 718.1, what is the least total drop in elevation from one end of the sewer to the other?

  1. 5 in
  2. 10 in
  3. 20 in
  4. 40 in
Show the answer

Answer: C. 2025 CPC 718.1 requires a building sewer to slope at least 1/4 in per ft, so 80 ft x 1/4 in = 20 in of drop. 10 in and 5 in use the reduced slopes of 1/8 and 1/16 in per ft, which need AHJ approval; 40 in uses 1/2 in per ft, steeper than the code minimum.

Source: 2025 California Plumbing Code 718.1, 2025 California Plumbing Code (Title 24, Part 5)

6. Your crew has laid a new PVC building sewer from the house to the public sewer connection, and it must be tested under 2025 CPC 723.1. Which test fits this plastic sewer?

  1. Run water from the house fixtures for 15 minutes and look for wet spots
  2. Pull a camera through it to check the joints, using no water or air
  3. Plug it at the public connection and fill it with water, lowest point to highest
  4. Plug it at the public connection and hold a low-pressure air test on it
Show the answer

Answer: C. 2025 CPC 723.1 tests a building sewer by plugging it at the public connection and filling it with water from the lowest to the highest point, or by a low-pressure air test, but the air test is not for plastic pipe, so a PVC sewer gets the water test and must be watertight. Running fixtures or a camera run does not prove it watertight.

Source: 2025 California Plumbing Code 723.1, 2025 California Plumbing Code (Title 24, Part 5)

Septic System Maintenance and Repair · 10% of the exam

Troubleshooting existing septic systems; removing or repairing existing septic systems; maintaining existing septic systems.

7. A homeowner calls because the toilets back up into the house. You find the tank was pumped last month and the inlet is clear, but the leach trenches are full of standing effluent that no longer soaks in. Under the State Water Board OWTS Policy, section 11.1 (corrective action), how is this system classed?

  1. Failing; the dispersal system needs a major repair to meet Tier 1, 2 or 3
  2. Failing; the septic tank needs a major repair to meet the Section 8 rules
  3. Not failing; a minor component repair returns it to Tier 0
  4. Not failing; pump the tank again and recheck the field in a year
Show the answer

Answer: A. Section 11.1 of the OWTS Policy says a system with wastewater backed up into plumbing fixtures, pooling or surfacing because its dispersal system no longer percolates is failing and needs a major repair: the dispersal system is replaced, repaired or modified to meet Tier 1, 2 or 3. A tank-only repair fits section 11.2 (baffle or structural failure), and a component fix fits section 11.3; here the problem is the field.

Source: OWTS Policy, Tier 4, section 11.1 (with 11.2 and 11.3), State Water Board OWTS Policy (Water Quality Control Policy for Siting, Design, Operation, and Maintenance of OWTS), amended 2023

8. A homeowner with a standard gravity septic system (no pumps, floats or other mechanical parts) asks how often a septic service professional should inspect it. What does EPA guidance for household systems say?

  1. At least every 5 years
  2. At least every 2 years
  3. At least every 3 years
  4. At least every 10 years
Show the answer

Answer: C. EPA's household septic guidance says the average household system should be inspected at least every three years by a septic service professional, and household tanks are typically pumped every three to five years. Alternative systems with pumps or floats are inspected more often, generally once a year; 2, 5 and 10 years are not EPA's inspection interval.

Source: EPA, How to Care for Your Septic System (inspect and pump frequently)

Safety · 27% of the exam

Employee training and safety; job site safety.

9. You are writing the Injury and Illness Prevention Program (IIPP) for your new septic installation company. Under Cal/OSHA 8 CCR 3203(a)(1), which item must the written program include?

  1. A federal OSHA inspector named to audit the program every year
  2. The person or persons with authority and responsibility for it
  3. Each employee's medical history and record of past injuries
  4. A state safety license number for every foreman on the crew
Show the answer

Answer: B. Cal/OSHA 8 CCR 3203(a)(1) makes the first IIPP element the identity of the person or persons with authority and responsibility for implementing the program. In California, Cal/OSHA, not federal OSHA, enforces the IIPP, and the rule does not call for medical histories or foreman license numbers.

Source: 8 CCR 3203(a)(1), Cal/OSHA Title 8, Section 3203 Injury and Illness Prevention Program

10. Your crew is digging a sewer lateral trench along a road shoulder, and the workers are exposed to public vehicle traffic. Under Cal/OSHA 8 CCR 1541(d), what must they be given and wear?

  1. Warning vests for the flagger only, not for the trench crew
  2. Warning vests only after dark, when drivers use their headlights
  3. Warning vests or garments of reflective or high-visibility material
  4. Reflective tape on hard hats, in place of any vest or garment
Show the answer

Answer: C. Cal/OSHA 8 CCR 1541(d) says employees exposed to public vehicular traffic shall be provided with, and shall wear, warning vests or other suitable garments marked with or made of reflectorized or high-visibility material. The rule is not limited to night work or to the flagger, and hat tape does not replace the garment.

Source: 8 CCR 1541(d), Cal/OSHA Title 8, Section 1541 Excavations, General Requirements

Where to go from here

How the license works, from experience to fees: how to get a C-42 license. Everything about the course: the C-42 exam page.

Questions from our free C-42 test, written from CSLB License Examination Study Guide, Sanitation System (C-42), form 13E-42 (03-2026), for examinations scheduled on or after June 1, 2023. CSLB notes that the percentages are approximate. Reviewed 14 October 2026. Study guidance only; we are not affiliated with CSLB or PSI.

The C-42 exam pageOutline, sample question, course and prices. Free C-42 test25 questions, scored by section. How to get a C-42 licenseExperience, exams, fees and pass rate. What a license costsEvery fee from application to renewal.