Five free practice questions on I/M emissions test failures: content area F of the ASE L1 Advanced Engine Performance Specialist test, 6 of its 50 scored questions (12%). They are in the ASE format: four options, one best answer. Answer each one before opening the explanation. No account, no email.
01The questions
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On a five-gas exhaust analyzer, which gas reading is the most direct sign that the air-fuel mixture is rich?
- Oxygen (O2)
- Carbon monoxide (CO)
- Oxides of nitrogen (NOx)
- Carbon dioxide (CO2)
Show the answer
B. Correct. Carbon monoxide forms when there is too little oxygen to burn the carbon in the fuel all the way to CO2, so it rises as soon as the mixture goes richer than stoichiometric. High CO with low O2 is the classic rich pattern.
Why not the others
A. Oxygen in the exhaust rises when the mixture is lean or a cylinder misfires, because air passes through unused. A rich mixture uses up the oxygen, so O2 falls; CO is the gas that rises.
C. NOx forms at high combustion temperatures and peaks slightly lean of stoichiometric. A rich mixture lowers combustion temperature and NOx; CO is the rich indicator.
D. CO2 is highest when combustion is complete near stoichiometric, so it falls rather than rises as the mixture goes rich. CO is the gas that climbs with excess fuel.
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A warm MAF-equipped engine fails a loaded-mode test for HC, with tailpipe O2 above normal and CO low. Any of these could cause the failure EXCEPT:
- a burned exhaust valve on one cylinder.
- a coil whose output fails under load.
- a PCV valve stuck closed.
- a partly clogged injector on one cylinder.
Show the answer
C. Correct. A PCV valve stuck closed adds no air to the charge and takes nothing from spark or fuel, so it cannot leave a cylinder unburned; its effect is crankcase pressure, with blow-by forced out another way. High HC with high O2 and low CO is a misfire pattern, which the other three can produce under load.
Why not the others
A. A burned valve leaks compression, so that cylinder burns poorly or not at all and sends HC and oxygen into the exhaust, the pattern these readings show.
B. Higher cylinder pressure under load takes more voltage to fire the plug, so a weak coil can misfire only then. A misfiring cylinder sends unburned fuel and unused oxygen out, raising HC and O2 with CO low.
D. A clogged injector starves its cylinder of fuel, and the lean charge can misfire under load, raising HC and O2 while CO stays low.
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The analyzer accepts a sample only if CO plus CO2 is at least 6.0%. A vehicle with one tailpipe reads at idle: HC 60 ppm, CO 0.10%, CO2 5.4% and O2 9.8%. What should the technician do?
- Look for an exhaust leak or a loose probe
- Diagnose a lean condition from the high O2
- Record the low HC and CO as a pass
- Replace the converter for low CO2 output
Show the answer
A. Correct. CO plus CO2 is 0.10 + 5.4 = 5.5%, under the 6.0% minimum, and the 9.8% O2 shows outside air is diluting the sample. An exhaust leak ahead of the probe or a probe not fully inserted must be fixed before the readings mean anything.
Why not the others
B. A lean engine raises O2 somewhat, but the CO plus CO2 total of 0.10 + 5.4 = 5.5% is below the 6.0% the analyzer needs, so this sample is diluted with outside air. Find the leak or probe problem first.
C. The sample fails the dilution check, 0.10 + 5.4 = 5.5% against a 6.0% minimum, so the low HC and CO may only be diluted. The sample is invalid until the leak or probe problem is fixed.
D. A converter raises CO2 by oxidizing CO and HC; it cannot make the CO plus CO2 total fall below what the engine produced. Outside air is diluting the sample, so fix the leak or probe and retest.
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A pickup fails a loaded-mode test for both HC and NOx. At the failing mode the analyzer reads HC 380 ppm, CO 0.15%, CO2 12.4%, O2 2.6% and NOx 1,350 ppm, and no DTCs are stored. What should the technician do first?
- Clean the EGR passages, then retest
- Find the misfire, then recheck NOx
- Replace the catalytic converter
- Correct a rich mixture first
Show the answer
B. Correct. High HC and O2 with low CO means a cylinder is passing its charge unburned, and that extra oxygen in the converter's feed gas stops it reducing NOx, so one fault can fail both gases. Repair the misfire, then recheck NOx under the same load before looking at EGR or the converter.
Why not the others
A. EGR is a common cause of high NOx, but high HC with 2.6% O2 and low CO is a misfire pattern, and the oxygen a misfire sends into the converter keeps it from reducing NOx. Find the misfire first, then measure NOx again before any EGR work.
C. A converter cannot reduce NOx in exhaust carrying 2.6% oxygen, new or old, and a misfire left in place can overheat a new one. The readings point to a misfire that must be fixed first.
D. A rich mixture raises CO and drives O2 down; here CO is 0.15% and O2 is 2.6%. The exhaust is oxygen-rich, a misfire pattern, so there is no rich condition to correct.
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Why does a loaded-mode emissions test, with the vehicle driven on a dynamometer, give a far more useful NOx reading than a no-load idle test?
- The EGR valve is wide open at idle, hiding NOx
- The NOx cell needs high exhaust flow to read
- The converter stores NOx at idle and releases it later
- The heat that forms NOx comes mainly under load
Show the answer
D. Correct. Nitrogen and oxygen combine in quantity only at very high combustion temperatures, which an engine reaches under load and not at a no-load idle. Driving against the dynamometer's load produces the NOx the engine makes on the road.
Why not the others
A. EGR is normally closed at idle, where it would make the engine run rough; it opens at part throttle under load. Idle NOx is low because combustion temperature is low.
B. The analyzer measures the concentration in the sample it draws, whatever the exhaust flow. Idle NOx is low because the engine makes little there; it forms at the high combustion temperatures of a loaded engine.
C. A three-way converter does not store NOx at idle for later release. Idle NOx is low because combustion temperatures are low without load.
02Answer key
| Question | Answer |
|---|---|
| Q1 | B |
| Q2 | C |
| Q3 | A |
| Q4 | B |
| Q5 | D |
03What area F covers
This content area covers failed I/M (inspection and maintenance) emissions tests: reading HC, CO, NOx, CO2 and O2 results and choosing the next test, HC and CO failures on no-load tests, HC, CO and NOx failures on loaded-mode dynamometer tests, OBD-based inspections with MIL, readiness and communication problems, and functional checks such as the fuel cap test.
The ASE test specification calls this content area “I/M Failure Diagnosis”. It carries 6 of the 50 scored questions, 12% of the test.
- Read I/M test results and how the gases relate
- Choose the next test from the five gas readings
- Diagnose HC and CO failures on no-load tests
- Diagnose HC, CO and NOx failures on loaded-mode tests
- Use test instruments to read electrical signals
- Check MIL operation and monitor readiness for OBD tests
- Solve communication failures with inspection equipment
- Run functional tests such as the fuel cap test
04What the questions turn on
What the gases say
High HC and O2 with low CO is a cylinder passing its charge unburned, such as a misfire. High CO with low O2 is the rich pattern: CO forms when there is too little oxygen to burn the carbon all the way to CO2.
Check the sample first
Before diagnosing the engine, confirm the sample is valid. When CO plus CO2 falls below the analyzer's minimum and O2 is high, outside air is diluting the sample. An exhaust leak or a loose probe is fixed before the readings mean anything.
One fault, two failed gases
NOx forms at the high combustion heat of a loaded engine, so loaded-mode tests measure it. A misfire sends oxygen into the converter and keeps it from reducing NOx, so high HC, O2 and NOx together call for fixing the misfire first, then rechecking NOx.
When the inspection cannot connect
Composite vehicle OBD inspection questions turn on which modules still talk. A running engine and working cluster show the bus between modules is up, so a DLC reaching no module is cut off from it. A cluster that loses the bus lights the MIL itself.
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.