TEST 2: Manually Creating A Rich Condition To Test The O2 Sensor
In this test section we're going to force a rich air/fuel mixture and see if the O2 sensor reacts to it (or not). And if it does react to this rich condition, we need to see how fast it does.
The fastest and easiest way to induce a rich air/fuel mixture is by spraying a little starting fluid into the engine while it's running. My preferred method is to spray starting fluid into a vacuum hose.
Once the starting fluid hits the engine cylinders, you'll get an instant rich air/fuel mixture. The oxygen sensor should react by immediately producing its maximum voltage (0.900 Volts +).
Alright, these are the test steps:
Start your Honda and let it idle for about 15 minutes, since you need a warmed up engine to get the O2 sensor to activate.
The multimeter should still be tapped into the signal wire from TEST 1.
The voltage numbers of the O2 sensor should moving between 0.100 and 0.900 Volts constantly on your multimeter's display after the engine has been running for 15 minutes.
If the voltage value stays fixed, don't worry about this yet, continue to the next step.
With the engine running, spray a little starting fluid into a vacuum hose (that has engine vacuum) while you observe your multimeter's display screen.
If you spray too much, there's a good chance that the engine will stall. If it happens, just restart the engine and repeat the step and spray less starting fluid.
The oxygen sensor should immediately report 0.800 to 0.900 Volts, as you spray the short bursts of carb cleaner into the vacuum hose. And as long as you're spraying (without the engine stalling), the voltage should stay there.
When you stop spraying, the O2 sensor values should come down and within a few seconds, they should start oscillating between 0.100 Volts to 0.900 Volts.
Let's take a look at what your test results mean:
CASE 1: The oxygen sensor voltage numbers immediately spiked to 0.900 Volts when you sprayed carb spray into the vacuum hose. This tells you that the oxygen sensor is OK at this point in time. It does not need to be replaced, since whatever's causing the computer to think it's fried is something else.
For more info on this, go to the section: Oxygen Sensor Codes Keep Coming Back.
CASE 2: The oxygen sensor voltage DID NOT react when you sprayed carb spray into the vacuum hose. This confirms that oxygen sensor is no longer working. You can replace the oxygen sensor.
CASE 3: The oxygen sensor voltage numbers SLOWLY spiked to 0.800 Volts after you sprayed carb spray into the vacuum hose. This confirms that oxygen sensor is too old and too slow. You should replace the oxygen sensor.
TEST 3: Manually Creating A Lean Condition To Test The O2 Sensor
Since you've verified that the oxygen sensor can react to a rich air/fuel mixture and report it, in this section we will induce a lean air/fuel mixture. We can do this easily by:
- Disconnecting a large vacuum hose.
- The vacuum hose we're gonna' disconnect is the one that connect to the brake booster.
Once this large vacuum hose is disconnected (with the engine running), the air/fuel mixture will instantly become lean. This will cause the oxygen sensor to produce its minimum voltage (between 0.100 to 0.200 Volts). We'll be able to see this low voltage reading on your multimeter's display.
As mentioned above, the hose that we'll disconnect is the one that connects the intake manifold to the vacuum brake booster. You don't have to remove it completely, just crack it open enough to get a good vacuum leak.
Alright, here are the test steps:
Crank and start your Honda Civic's engine. Let the engine run for about 15 minutes.
Your multimeter should still be tapped into the oxygen sensor's signal wire from TEST 1.
Disconnect vacuum hose with the engine running as you watch the reading of the O2 sensor's voltage reading on your multimeter's display.
If the engine is stalls, simply restart it and start from step 1 again (and disconnect the hose just a little this time).
As you are letting air enter the brake booster's vacuum hose, the voltage reading of the O2 sensor should decrease to about 0.100 to 0.200 Volts. And while the vacuum hose is letting ambient air into the intake manifold, the voltage should remain at about 0.100 to 0.200 Volts.
Reconnect the brake booster's hose and stop the vacuum leak. The voltage reading should now start to move up and down between 0.100 to 0.900 Volts.
Let's take a look at what your test results mean:
CASE 1: The voltage immediately dropped to 0.100 Volt as you opened the brake booster's vacuum hose. This tells you that the oxygen sensor is working well since it can react to a lean air/fuel mixture and report it.
Now, if the oxygen sensor passed the test in TEST 1 and passed this test (TEST 2), then you can conclude with certainty that the oxygen sensor is working properly and it doesn't need to be replaced. If the computer is still accusing it of being defective, take a look at this section: Oxygen Sensor Codes Keep Coming Back.
CASE 2: The voltage DID NOT drop to 0.100 Volt as you opened the brake booster's vacuum hose. This test result tells you that the oxygen sensor is defective since it can not react to a lean air/fuel mixture. Replacing the O2 sensor will solve the issue.