TEST 3: Manually Inducing A Lean Air/Fuel Mixture
A common mistake many folks make is assuming that a P0420 code points to a bad rear oxygen sensor. They replace it, only to see the P0420 code and check engine light come back in a few days.
Fortunately, there are two simple tests that allow us to determine if the rear oxygen sensor is really fried. The first test involves creating a lean air/fuel mixture and observing the sensor's reaction to this condition.
The second test involves inducing a rich air/fuel mixture and again analyzing the sensor's reaction (this second test will be carried out in TEST 4).
Both of these tests involve observing the voltage reading generated by the rear oxygen sensor using a scan tool with 'Live Data' functionality.
CAUTION: While performing this test, the engine will be running. Be extremely careful around any moving parts to avoid injury.
Let's get started:
- 1
Crank and start your Neon's engine. Let the engine idle for about 15 minutes.
- 2
Connect your scan tool to your vehicle's diagnostic port.
- 3
Enter the Live Data function.
- 4
Scroll down to the rear oxygen sensor voltage reading. This reading is generally labeled as O2S12.
The O2S12 reading will show you the voltage activity of the rear oxygen sensor. - 5
After 15 minutes of the engine running, the voltage reading should oscillate between 0.100 and 0.500 Volts every few seconds.
If the reading shows a fixed value (that neither rises nor falls), don't worry. Proceed to the next step. - 6
Disconnect a large vacuum hose with the engine running while watching the O2S12 reading on your scan tool's screen.
NOTE: If the engine shuts off, simply restart it and begin from step 1 (and disconnect the hose only slightly this time). - 7
As you're letting air in (or letting vacuum escape), the O2S12 voltage reading should drop to 0.100 to 0.200 Volts. And while the hose is open or disconnected (and depending on the amount of the vacuum leak), the voltage should stay between 0.100 to 0.200 Volts.
- 8
Shut off the vacuum leak.
- 9
The voltage reading should start to oscillate between 0.100 Volts to 0.500 Volts within a couple of seconds after shutting off the vacuum leak.
Let's take a look at what the test results mean:
CASE 1: The voltage dropped to 0.100 Volts when opening (disconnecting) the vacuum hose. This indicates that the oxygen sensor is working correctly and is not faulty (as it can react to the lean air/fuel mixture and report it).
The next step is to now check that the oxygen sensor can react and report a rich air/fuel mixture. For this test, go to: TEST 4: Manually Inducing A Rich Air/Fuel Mixture.
CASE 2: The voltage did NOT drop to 0.100 Volts when opening (disconnecting) the vacuum hose. This result indicates that the oxygen sensor is faulty and needs to be replaced.
If the rear oxygen sensor were operating correctly, it would have reacted immediately upon disconnecting the hose and generated a voltage of around 0.1 Volts (100 milliVolts). Since it didn't, you can correctly conclude that it's faulty and needs to be replaced.
CASE 3: The wiring of the rear oxygen sensor is burned and stuck to the exhaust pipe: Replace the rear oxygen sensor with a new one.
TEST 4: Manually Inducing A Rich Air/Fuel Mixture
In this last rear O2 sensor performance test, you'll manually induce a rich air/fuel mixture. Unlike the previous test, the procedure for inducing a rich air/fuel mixture is a whole lot simpler.
What you'll do is spray a small amount of starting fluid into the throttle body while the engine is running.
If you have a 1995-1999 Neon, you'll remove the air duct that connects to the throttle body and spray starting fluid into the throttle body itself (with the engine running).
If you have a 2000-2005 Neon, you'll need to remove the top part the air filter box and remove the air filter. You'll spray starting fluid into the intake air opening at the bottom of the air filter box (with the engine running).
CAUTION: While performing this test, the engine will be running. Be extremely careful around any moving parts to avoid injury.
These are the steps:
- 1
1994-1999 2.0L SOHC Neon: Remove the air duct hose from the throttle body (no need to touch the air filter).
2000-2005 2.0L SOHC Neon: Remove the air filter box cover and remove the air filter. - 2
Start the engine.
NOTE: For the accuracy of the test, let the engine idle for about 15 minutes before inducing the rich air/fuel mixture (in step 7). - 3
Connect your scan tool to your vehicle's diagnostic port.
- 4
Enter the Live Data function.
- 5
Scroll down to the rear oxygen sensor voltage reading (generally labeled as O2S12).
The O2S12 reading will show you the voltage activity of the rear oxygen sensor. - 6
After 15 minutes of the engine running, the voltage reading should oscillate between 0.100 and 0.900 Volts every few seconds.
If the reading shows a fixed value (that neither rises nor falls), don't worry. Proceed to the next step. - 7
Spray a bit of starting fluid into the throttle body (or air filter box) with the engine running while watching the O2S12 reading on your scan tool's screen.
If you spray too much starting fluid, the engine will shut off. If this happens to you, simply restart the engine and start from step 1 (spraying less starting fluid this time).
As you're spraying the starting fluid, the voltage reading of the rear O2 sensor should rise to 0.800 to 0.900 Volts. - 8
Stop spraying the starting fluid.
- 9
The voltage reading should start to oscillate between 0.100 Volts to 0.900 Volts within a couple of seconds.
Let's take a look at what the test results mean:
CASE 1: The voltage spiked to 0.900 Volts when spraying starting fluid into the vacuum hose. This is the correct and expected result.
You can conclude that the catalytic converter is bad and is the cause of the P0420 code if you have:
- Confirmed a P0420 diagnostic trouble code is stored in the computer's memory.
- Confirmed that the rear oxygen sensor reacts to a manually induced lean air/fuel mixture (TEST 3).
- Confirmed that the rear oxygen sensor reacts to a manually induced rich air/fuel mixture (this test section).
CASE 2: The voltage did NOT spike to 0.900 Volts when spraying starting fluid into the vacuum hose. This result tells you that the rear oxygen sensor is faulty and you need to replace it.
If this info saved the day, buy me a beer!
