AF1 Alternate Fuels — CNG — practice test

High-yield AF1 topics

The material that shows up year after year. If you're short on time, start here.

• Container types and life limits. Know Type 1 (all metal) through Type 4 (all composite over a plastic liner) and what damage each tolerates. The FMVSS 304 label carries a hard expiration date, service pressure, and manufacture date, and NFPA 52 drives the recurring visual inspection (commonly every 36 months or 36,000 miles, and after any collision or fire) by a certified CNG fuel system inspector. Know CGA C-6.4 damage Levels 1, 2, 3 and what condemns a cylinder: cut/abrasion depth into the wrap, UV chalking, battery acid or road salt on an aluminum liner, any weld ground or arc strike near the tank.
• PRDs and vent lines. CNG PRDs are thermally activated (eutectic plug around 212 F) often paired with a rupture disc. They are one-shot: once a PRD has vented, it gets replaced, never reset or reused. Vent tubing has to route to the outside, upward, away from the occupant compartment, wheel wells, and exhaust, and cannot be capped, kinked, or dented. Expect questions on where the vent terminates and on Type 4 tanks needing PRD coverage at both ends.
• Pressure staging and regulator icing. Full service pressure is 3,600 psi settled at 70 F (older systems 3,000 psi), stepped down at a high-pressure regulator with a coalescing filter to an intermediate stage, then to the injector rail around 100-150 psi. That expansion is Joule-Thomson cooling, which is why the regulator is coolant heated. Air in the cooling system, a lazy thermostat, or a restricted heater feed will ice the regulator and give you lean codes, hesitation, and stalls under load. Do not chase that as an injector problem.
• Density correction, fuel gauge, and hot fills. The ECM does not measure volume, it calculates mass from fuel rail pressure plus fuel temperature (PV=nRT). A skewed fuel temp sensor makes the strategy rich or lean with no mechanical fault present. The dash gauge is a pressure transducer, not a float, so a truck that reads down on a cold morning is normal. A fast fill heats the gas by compression, so it reads full on the dispenser and settles short once it cools. Temperature-compensated dispensers exist for exactly this reason.
• Engine-side differences from gasoline. Methane has a slow flame speed, so it needs more spark advance; it is dry and high pressure at the gap, so plug gaps run tight (roughly 0.020-0.025 in) with a high-output coil, and worn plugs cause misfire long before a gasoline engine would care. No liquid fuel means no valve seat lubrication, so hardened seats and valve recession checks matter. Use low-ash oil (CES 20074 or equivalent) or you get valve deposits and a plugged cat. Aftertreatment is a methane oxidation catalyst with high palladium content and a high light-off, not a plain three-way.
• Leak detection and defueling procedure. Approved electronic combustible-gas detector or an approved leak detection solution only. Never a flame, and never a soap product containing ammonia because it attacks brass fittings. Shop-side: methane detectors mount at the ceiling, ventilation is high-point, and defueling happens at an approved vent stack outdoors with the vehicle bonded, never into a bay. Know the 1/4-turn manual tank valve, the solenoid, and the excess-flow function and what each one does during a leak event.

Where techs lose points on AF1

Good technicians miss these — not from lack of skill, but because the question is built to catch them.

• Propane logic on a methane test. Methane has a specific gravity around 0.6, so it rises and disperses; LPG is heavier than air and pools. Every distractor built on low-point ventilation, floor-level detectors, checking a pit, or leaks collecting in a low spot is wrong on AF1. Same trap in reverse: flammability limits are roughly 5-15 percent and autoignition is around 1,000 F, well above gasoline, so 'a small leak near a hot exhaust ignites like gasoline' is not the answer they want. Also do not trust your nose. Pipeline gas is odorized, but CNG at some fleet stations is not reliably odorized, so smell is never the approved detection method.
• Thinking a CNG cylinder can be recertified. Techs who have handled SCBA bottles or DOT cylinders assume a hydro test buys you another cycle. It does not. An NGV cylinder is life-limited by the FMVSS 304 label date and comes out of service when it expires regardless of how clean it looks or how well it passed a visual. If the answer choice extends service life through testing, it is the wrong one.
• Blaming the tank or dispenser for a short fill. A hot fast fill on a summer day reads full at the dispenser and settles low an hour later. That is heat of compression, not a leak, not a bad transducer, not a failed cylinder. Same idea with the dash gauge reading down on a cold morning: it is a pressure sensor, not a float, and pressure tracks temperature. Both scenarios show up with tempting 'replace the sensor' or 'find the leak' distractors.
• Wrong tools and wrong fittings on the high-pressure side. A halogen or refrigerant sniffer will not find methane, and household soap with ammonia in it attacks brass fittings even though it bubbles just fine. On assembly, do not reach for Teflon tape on cone-and-thread or metal-to-metal seals, do not reuse a kinked or previously bent stainless line, and do not reuse a one-time-use gasket. If a question offers 'reset the PRD' as a choice, it is bait: thermally activated PRDs get replaced.

A study plan that works for AF1

Roughly two weeks of real preparation, in the order that actually builds on itself.

1. Days 1-4: Codes and containers first, because that is where the free points are and where most techs are weakest. Read NFPA 52 sections on containers, PRDs, and vent lines, then the CGA C-6.4 damage level definitions. Learn to read an FMVSS 304 label cold: service pressure, manufacture date, expiration date. Drill Type 1 through Type 4 construction and failure modes until you can name them from a description.
2. Days 5-8: Fuel system flow path. Draw it from the fill receptacle (NGV1) through the check valve, cylinder valve, manual 1/4-turn, solenoid, high-pressure line, coalescing filter, high-pressure regulator, low-pressure regulator, and rail. Write the pressure at every point. Then work the diagnostics: where you place a gauge for a low-power complaint, what a restricted coalescing filter looks like versus a partially closed tank valve versus regulator icing.
3. Days 9-11: Engine performance and metering. Fuel rail pressure plus fuel temp equals calculated mass, so build every scenario around that. Practice lean/rich cases with a skewed fuel temp sensor, a coolant flow problem at the regulator, and a worn plug gap. Cover the methane oxidation catalyst, NMHC versus THC, low-ash oil, and valve seat recession.
4. Days 12-14: Procedure and traps. Walk through defueling, leak checking, cylinder inspection, and post-collision inspection out loud, step by step, like you are explaining it to a helper. Then take timed practice sets and audit every miss: if the wrong answer felt right, it is almost always because you applied a propane, DOT-cylinder, or gasoline rule. Write those down and reread them the morning of the test.

Sample AF1 questions

Straight from the bank — answers highlighted, with the explanation underneath.