CAUTION: Ignition must be turned OFF before disconnecting or connecting wires or test equipment in system.

Basic tune-up specifications must be correct. See appropriate TUNE-UP article in the TUNE-UP section under the ENGINE PERFORMANCE category. Duty cycle dwell reading with oxygen sensor connected must fluctuate within range shown in Chart 1. See Fig 1 . Range at sea level should be 14-32°. At 3000 feet, dwell range should be 25-43°. At 6000 feet, dwell range should be 36-54°.

Fig 1: Duty Cycle (Dwell) Charts Use Charts 1-4 as described in text.
Check CO% and HC readings before proceeding with trouble shooting procedure. CO range is .3-1.2%. HC maximum reading is 400 PPM. If all readings are correct and vehicle runs poorly, problem is usually not in carburetor or oxygen sensor system. If readings are incorrect, adjust duty cycle.
If unable to adjust system properly, proceed with trouble shooting procedure. Duty cycle open-loop test is first. Disconnect Green wire to oxygen sensor. Do not ground wire. Duty cycle should match dwell readings in Chart 1 but be steady rather than fluctuating. If duty cycle does not give steady reading, proceed to ECU test.
If duty cycle gives steady reading, proceed to cold running enrichment test. With oxygen sensor disconnected and oxygen sensor system temperature sensor (in coolant hose) lead disconnected, duty cycle dwell should match range in Chart 4. Reading should be steady. If dwell reading incorrect and not steady, proceed to ECU test.
If reading correct and steady, perform rich mixture limit check. Reconnect oxygen sensor system temperature sensor. Ground Green wire that leads to oxygen sensor. Duty cycle dwell should match range in Chart 2 and CO% reading must increase. If duty cycle reading is incorrect, proceed to ECU test.
If duty cycle reading is correct but CO% reading does not increase, test feedback solenoid (frequency valve) operation. With ignition key on, solenoid should click about 10 times per second. If not, check wiring for power and ground. If wiring good, replace frequency valve.
If duty cycle reading is correct and CO% reading increases with wire grounded, perform open-loop CO test. With Green wire to oxygen sensor disconnected but not grounded, CO reading must be in range of .3-1.2%. If incorrect, CO% must adjust to range of .8-1.2%.
If CO% reading correct or adjustable, replace defective oxygen sensor. If CO% reading cannot be adjusted to correct range, check engine for mechanical problem in valve train, ignition system, or compression reading. Correct engine problem as necessary.

When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

• You smell fuel, burning insulation, or see smoke.

• Brakes feel soft, pull hard to one side, or make grinding noises.

• The engine overheats, stalls repeatedly, or misfires under load.

• You are missing required tools, torque specs, or safe lifting equipment.

• You are not confident in the next step or safety outcome.

Safety Warning

Oxygen Sensor

Use the Manual With the Right Hardware

When to See a Mechanic

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