CATS Aftertreatment & Emissions — practice test

High-yield CATS topics

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

• Passive vs active regen, cold. Passive is NO2-driven soot oxidation using NO2 the DOC made from NO, running on its own roughly 250-350 C with no added fuel and no dash message. Active is commanded: post-injected fuel is oxidized ACROSS the DOC to drive DPF bed temp to about 550-600 C. They ask this four different ways, including the Tech A/Tech B claiming the fuel burns in the cylinder to heat the filter. It does not.
• DEF spec and cold behavior. 32.5% urea / 67.5% deionized water, ISO 22241. Freezes at about -11 C (12 F), and freezing does NOT ruin it - urea and water freeze together and it thaws back to concentration. The system uses coolant-heated tank and lines and simply does not dose until thawed. Storage is stainless or approved HDPE; carbon steel, brass, copper, and aluminum are wrong answers that look reasonable.
• SCR conversion efficiency = inlet NOx sensor vs outlet NOx sensor, period. Know that the downstream sensor is cross-sensitive to ammonia and reads NH3 slip as if it were NOx - that is how over-dosing shows up as a low-efficiency fault. Know that both sensors are heated, that the ECM delays the heater until past the exhaust dew point so water droplets do not crack the ceramic element, and that you cannot bench-check one with an ohmmeter.
• Soot vs ash, and which way delta-P lies. Soot is carbon and burns off in regen; ash is metallic residue from lube-oil additives (calcium, zinc, sulfated ash) and never burns. A 400,000-mile DPF that will not come back down after repeated successful regens is ash-loaded - clean or replace, do not force another regen. Opposite direction: a plugged delta-P line reads LOW, and near-zero delta-P at high load and full flow is a dead sensor or line, not a clean filter.
• Inducement staging and SPN/FMI. Escalation is lamp/warning to torque derate (roughly 25%) to the final low-speed limit, about 5 mph, when DEF is empty or quality has persistently failed. SPN names the component or parameter; FMI names the failure type. Read a code like SPN 4364 FMI 18 as SCR conversion low, not as a part number to throw at the truck.
• EGR: cooler and valve. Internal cooler leak signature is coolant loss with no external leak, white sweet-smelling vapor, no oil contamination - pressure-test the cooler instead of chasing a head gasket. Know the cooler exists to lower charge temp and cut thermal NOx, that EGR works by diluting the intake charge with inert gas, and that the EGR delta-P sensor infers mass flow across an orifice/venturi.

Where techs lose points on CATS

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

• The refractometer trap. 32.5% is in-spec per ISO 22241, but a refractometer reads index of refraction - nothing else. DEF cut with diesel, coolant, or a random additive can still read 32.5% and still poison the catalyst. In-spec reading plus a recent fuel-island fill plus a conversion fault means pull a sample, not clear-and-release.
• Blaming the DPF for frequent regens. When a truck regens far more often than normal with no DPF crack or dosing code, the filter is doing its job - something upstream is making excess soot. Leaking injectors, a tired turbo, air-intake restriction, or an EGR valve stuck open. Replacing the DPF puts the truck right back in the bay.
• Low-conversion fault with inlet and outlet NOx sensors reading nearly identical. The tempting answer is a dead SCR catalyst. Identical readings mean no reduction is happening at all, which points at reductant never reaching the catalyst - no dosing, plugged injector, crystallization in the decomposition tube - before the brick itself. Confirm actual dosing before condemning a catalyst.
• Hard white crystalline deposits at the dosing injector or decomposition tube. Techs read that as bad DEF. It is a thermal/spray problem: exhaust too cool at the dosing point, poor atomization, wrong spray targeting, or a dribbling injector. Also watch the EXCEPT items on DEF contamination - contamination does not cause every symptom on the list, and hard crystals are the usual bait.

A study plan that works for CATS

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

1. Days 1-4: SCR/DEF end to end, because it is the biggest slice. Chemistry (32.5/67.5, ISO 22241, -11 C / 12 F freeze and thaw strategy), dosing unit and airless metering, decomposition tube and mixer, catalyst, ammonia slip catalyst. Finish by writing the full inducement escalation from memory - lamp to 25% derate to 5 mph - and the two-NOx-sensor efficiency calculation.
2. Days 5-8: DPF and regen. Drill passive vs active until you can state the mechanism (NO2 oxidation vs post-injected fuel oxidized in the DOC) and the temperature targets (~250-350 C vs ~550-600 C) without pausing. Then soot vs ash, both soot-estimation methods (pressure model plus time/fuel-based model), the full list of regen inhibitors, and parked-regen safety.
3. Days 9-11: EGR, sensors, and code reading. EGR cooler leak signature and pressure test, stuck-open EGR symptoms, the delta-P flow model across the orifice or venturi. NOx sensor dew-point heater delay and NH3 cross-sensitivity. Practice reading SPN/FMI pairs out loud until you split component from failure mode automatically.
4. Days 12-14: Nothing but composite and EXCEPT questions in the CATS bank. Score every miss into one of two buckets - I did not know it, or I got baited. The baited pile is your real study list. Re-read those explanations the morning of the test and go take it.

Sample CATS questions

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