Five free practice questions on emission control diagnosis: content area E of the ASE L1 Advanced Engine Performance Specialist test, 8 of its 50 scored questions (16%). They are in the ASE format: four options, one best answer. Answer each one before opening the explanation. No account, no email.
01The questions
-
A technician is inspecting a vehicle for a removed or gutted catalytic converter. Any of these findings could point to tampering EXCEPT:
- a spacer between the downstream HO2S and the pipe.
- a downstream HO2S signal that stays fairly steady.
- an MIL that never lights at the key-on bulb check.
- an empty shell seen through the upstream sensor port.
Show the answer
B. Correct. A downstream signal that stays fairly steady while the upstream sensor switches is what a working converter with oxygen storage produces. It is the normal reading, not a sign of tampering, unless a spacer is found damping it.
Why not the others
A. A spacer pulls the downstream sensor out of the exhaust stream so it reports a steady, damped signal even with no working catalyst ahead of it. It could point to tampering.
C. With a converter removed or gutted, the catalyst monitor would turn on the MIL, so a lamp that never lights even at its key-on check may have been disabled to hide that. It could point to tampering.
D. A borescope view through the upstream sensor port that shows no substrate means the catalyst has been removed. It could point to tampering.
-
An EVAP purge valve sticks closed on a vehicle that is otherwise in good condition. Which of these emissions increases as a result?
- Vapor escaping at the canister vent
- HC at the tailpipe at idle speed
- CO at the tailpipe on hard acceleration
- NOx at the tailpipe at highway cruise
Show the answer
A. Correct. With no purge flow the canister is never cleaned out; once its charcoal is full, tank vapor passes through it and out the vent to the air. Evaporative HC rises while tailpipe emissions stay the same.
Why not the others
B. With purge closed no vapor reaches the engine, so tailpipe HC does not rise. The vapor the canister can no longer release ends up escaping through its vent.
C. Tailpipe CO depends on combustion and the converter, and a closed purge valve adds no fuel to either. The added emission is evaporative, through the canister vent.
D. A closed purge valve does not change combustion temperature or oxygen, so NOx is unaffected. The emission that rises is fuel vapor escaping the saturated canister.
-
A warm engine with normal fuel trims and a switching upstream oxygen sensor is held at 2,500 rpm. The manufacturer's procedure calls for the converter outlet to read at least 50°F hotter than its inlet. An infrared thermometer reads 610°F at the inlet and 625°F at the outlet. What does this show?
- The converter is working normally
- The converter is oxidizing too little
- The engine is running too rich
- The converter is restricting exhaust flow
Show the answer
B. Correct. Oxidizing HC and CO releases heat, so a working converter's outlet runs hotter than its inlet; here the rise is 625 - 610 = 15°F against the procedure's 50°F minimum. With fuel control normal, the converter is not doing its job.
Why not the others
A. The outlet is only 625 - 610 = 15°F hotter than the inlet, well short of the 50°F rise the procedure calls for. The converter is not oxidizing enough HC and CO.
C. Normal trims and a switching upstream sensor show the mixture is under control. With fuel control normal, the low temperature rise points to the converter itself.
D. A temperature rise measures how much the converter oxidizes, not how freely exhaust flows through it. Restriction is checked with a backpressure or vacuum test; this reading shows too little conversion.
-
An older pickup with a positive-backpressure EGR valve pings under load and has an EGR flow DTC. Vacuum from the control solenoid reaches the valve's hose when commanded. With the engine off, a hand vacuum pump on the valve will not hold vacuum, and the diaphragm does not move. What should the technician do next?
- Replace the EGR valve for a leaking diaphragm
- Repeat the test with the engine running
- Clean carbon from the EGR passages
- Test the EGR control solenoid's circuit
Show the answer
B. Correct. A positive-backpressure valve has an internal bleed that only exhaust backpressure closes, so with the engine off it vents the applied vacuum and the test cannot be read. With the engine running fast enough to build backpressure, applied vacuum should open the valve and pull engine speed down; if it does not, the valve or its passages are at fault.
Why not the others
A. On a positive-backpressure valve a bleed stays open until exhaust backpressure closes it, so failing to hold vacuum with the engine off is normal. The valve has not yet been shown to be faulty.
C. Passage cleaning is a repair for a valve that opens but flows nothing. This valve has not been tested under the conditions it needs, so the engine-running test comes first.
D. The stem already shows the solenoid delivering vacuum to the valve's hose when commanded, so its circuit works. The valve itself needs to be tested with the engine running.
-
A manufacturer's strategy holds EVAP purge at 0 percent after a cold start until the engine enters closed loop. What is the reason?
- Only closed-loop feedback can correct for the vapor
- Fuel vapor will not burn in a cold combustion chamber
- Purge vapor would overheat the cold converter
- Manifold vacuum is too low to pull vapor when cold
Show the answer
A. Correct. Purge vapor adds an unknown amount of fuel, from almost none to a lot after refueling, and the PCM can only correct for it by watching the oxygen sensors in closed loop. Purging in open loop would let the mixture swing rich or lean.
Why not the others
B. Fuel vapor burns readily even in a cold engine; vapor is in fact easier to burn than liquid fuel. The purge delay is about fuel control, not combustibility.
C. A cold converter has not reached light-off, so it is not at risk of overheating from purge vapor. Purge is held off until closed loop so the PCM can trim for the unknown vapor.
D. A cold engine at idle still has ample manifold vacuum to pull vapor, so purge flow is not limited by vacuum. Purge waits for closed loop so the oxygen sensor feedback can correct for the vapor.
02Answer key
| Question | Answer |
|---|---|
| Q1 | B |
| Q2 | A |
| Q3 | B |
| Q4 | B |
| Q5 | A |
03What area E covers
This content area covers the emission control subsystems: PCV, ignition timing control, idle and deceleration control, EGR, the catalytic converter, secondary air injection and EVAP with its ORVR and engine-off natural vacuum features. It covers functional tests on each one, how a failed part changes tailpipe emissions, exhaust gas analyzer readings, and telling an emission system fault from another engine management fault.
The ASE test specification calls this content area “Emission Control Systems Diagnosis”. It carries 8 of the 50 scored questions, 16% of the test.
- Separate emission control faults from other engine faults
- Diagnose PCV, EGR and secondary air systems
- Test catalytic converters and judge their efficiency
- Diagnose EVAP, ORVR and engine-off leak testing
- Check ignition timing, idle and deceleration controls
- Predict how a failed part changes tailpipe emissions
- Use exhaust gas analyzer readings to find failures
- Find root causes and verify the repair
04What the questions turn on
Reading the five gases
A converter that stops working lets HC, CO and O2 rise while CO2 drops slightly, since CO2 is what it makes. High O2 with low CO2 while HC, CO and trims are normal means outside air is diluting the exhaust ahead of the probe.
Fix what killed the part
A part that keeps failing may be a victim. Oil burned in the cylinders coats a new converter with ash and phosphorus until it fails again, and a canister shedding charcoal granules holds each new purge valve open.
EGR at the wrong time
EGR is normally closed at idle, so EGR that will not flow brings knock, NOx under load or a flow DTC, not a rough idle. A coolant sensor falsely reading hot lets the PCM command EGR into a cold engine, which stumbles.
EVAP monitor conditions
Composite vehicle EVAP questions turn on the leak tests' enable criteria. A monitor that stays incomplete can be normal when one condition, such as fuel level, is never met. Clearing DTCs resets EVAP readiness to not complete until the test runs again.
05The other content areas
The L1 test has six content areas. Each has its own page of free questions:
The ASE L1 Advanced Engine Performance Specialist practice test page describes the full question bank. How these questions are written and checked: how our questions are made. Found a mistake? Tell us and we will fix it.
ToolboxPrep is an independent study site. It is not affiliated with or endorsed by ASE (the National Institute for Automotive Service Excellence). Our questions are original practice questions written to the published ASE test specifications; they are not actual test questions.