Testing The Front Oxygen Sensor (1995-2000 1.6L Honda Civic)

When the upstream oxygen sensor fails (commonly known as O2S11) on your OBD II equipped 1.6L Honda vehicle, you're gonna' see the check engine light lit up on your instrument cluster.

Not only that, you're gonna' spend a little more money at the pump since the your car won't get the same gas mileage it did before it failed.

Thankfully, the O2 sensor can be tested, before it's replaced, and in this article, I'll show you how.

ES You can find this tutorial in Spanish here: Cómo Probar El Sensor De Oxígeno (1.6L Honda Civic) (at: autotecnico-online.com).

APPLIES TO: This tutorial applies to the following vehicles:

• 1.6L Honda Civic: 1995, 1996, 1997, 1998, 1999, 2000.
• 1.6L Honda Civic Del Sol: 1994, 1995, 1996, 1997.

Important Suggestions And Tips

TIP 1: The info in this tutorial will help you to test the oxygen sensing performance of the front oxygen sensor. You won't find any info on how to test the front O2 sensor's heater or trouble code P0135. For the testing procedures of the front O2 sensor's heater, see this tutorial: How To Test Trouble Code P0135 (1995-2000 1.6L Honda Civic).

TIP 2: The test info in this article can only be applied if you have a scan tool. This scan tool must have Live Data capability.

You don't need the Honda factory scan tool or an expensive professional technician level scan tool to follow the test procedures in this article, since a simple generic scan tool will do just fine (Don't have a scan tool? Need a scan tool? Check out my recommendation: ZM301 OBD2 Scanner Diagnostic Tool (Amazon affiliate link).

TIP 3: The oxygen sensor test described in this tutorial is an on-car test, so you don't need to remove it to test it.

Symptoms Of A Bad Oxygen Sensor

The fuel injection computer in your Honda is constantly adjusting the air/fuel mixture that the engine needs to burn to create power, get good fuel economy, and pollute less.

The main component it uses to control (fine-tune) the air fuel mixture is the upstream oxygen sensor. Since the feedback information that the O2 sensor provides is so important, your Honda is gonna perform poorly when it fails.

The effects of a bad oxygen sensor can be very subtle since they usually don't cause serious drive-ability problems. Here are the most common symptoms:

• The O2 sensor will stay stuck reporting one voltage value as the engine runs (instead of producing a voltage that moves between 0.100 and 0.900 Volts).
  • It will stay stuck below 0.500 Volts. Reporting a never-ending lean condition.
  • It will stay stuck above 0.500 Volts. Reporting a never-ending rich condition.
• O2 sensor reacts to slowly to changes in the air/fuel mixture.
• The check engine light (CEL) will be shining nice and bright to let you know there's a problem.
• One of the following diagnostic trouble codes registered in the PCM's memory:
  • P0131 Upstream Heated Oxygen Sensor (O2S11) Circuit Out Of Range Low Voltage (Bank 1).
  • P0133 Upstream Heated Oxygen Sensor (O2S11) Circuit Slow Response (Bank 1).
• Really bad gas mileage.
• Won't pass state mandated emission testing.

Oxygen Sensor Basics

The oxygen sensor's job is to report back to the PCM (Powertrain Control Module) whether it's dumping in too much fuel or not enough.

Once the engine is running and fuel is being injected into each cylinder, the PCM has to constantly fine-tune the amount of fuel injection. It's always making little adjustments so your Civic squeezes out the best miles per gallon and, at the same time, keep pollution in check.

Here are some more specifics:

• As the engine runs, the PCM is constantly injecting fuel. Now, if it injects too much, the oxygen sensor reacts by sending out a higher voltage signal —anything above 0.500 Volts. In fact, it can climb as high as 0.900 to 1.0 volt.

  Anytime the PCM sees a voltage over that 0.500 mark, it knows the engine is running rich —basically too much fuel for the amount of air coming into the engine. So, it cuts back on fuel.

• But here's the thing: when the PCM trims the fuel back, it can go too far the other way. If it doesn't inject enough, that's a lean condition —not enough fuel for the air that's entering the engine.

  In a lean condition, the oxygen sensor's voltage drops below 0.500 Volts. And depending on how lean the mix is, you'll see the voltage sink down to about 0.050 to 0.100 Volts.

  Once the PCM sees that low voltage, it knows it has to add more fuel back in.

• This back-and-forth adjustment —rich to lean and lean to rich— goes on the whole time the engine is running (assuming the O2 sensor is alive and kicking).

• And the cool thing is, you can watch this dance happen with a scan tool in Live Data mode. That's exactly how I'm gonna' show you how to test it.

• A good oxygen sensor won't stay stuck. It'll constantly switch between rich and lean several times a second. So, if you see the voltage freeze in one spot while testing, that sensor is toast.

TEST 1: Checking The O2 Signal With A Scan Tool

The very first thing we need to do, after confirming that the check engine light (CEL) is lit up by oxygen sensor performance trouble codes, is see what the oxygen sensor is doing with the engine running.

This will tell us if: It's stuck reporting a rich condition, or stuck reporting a lean condition, or it's responding too slowly to changes in the air/fuel mixture. If the terms 'rich' and 'lean' aren't familiar to you, take a look at the section Oxygen Sensor Basics for more info.

You and I can see what the oxygen sensor is reporting by connecting a scan tool with live data capability and seeing what the O2S11 PID is reporting.

NOTE: If you don't have a scan with live data capability and need a recommendation, take a look at this one: ZM301 OBD2 Scanner Diagnostic Tool (Amazon affiliate link).

These are the test steps:

1. 1

   Start the engine and let it warm up to normal operating temperature.
   
   NOTE: If the engine's completely cold, bring it up to about 2,000 RPMs for around 5 minutes, till the upper radiator hose starts to feel warm to the touch.

2. 2

   Connect your scan tool and go to its Live Data function.

3. 3

   Find the Parameter ID (PID) for the front oxygen sensor.
   
   The upstream O2 sensor's PID is usually labeled as O2S11. Depending on the scan tool you're using, it might show up under a slightly different label —so check your scan tool's owner's manual to see exactly what it's called in your case.

4. 4

   Observe the front O2 sensor voltage changes on your scan tool once the engine has reached normal operating temperature and you've let it return to its normal idle RPM.

5. 5

   The front O2 sensor's voltage should constantly be changing between 0.4 and 1 Volt DC the entire time the engine's running.

Let's take a look at what your test results mean:

CASE 1: The O2 sensor signal voltage moved up and down the as the engine idled. This is the correct test result since it shows the oxygen sensor reacting and reporting the lean and rich conditions the fuel injection computer is creating and compensating.

There's a good chance that even tho' the voltage is moving up and down (as it should), the O2 sensor is reacting too slowly. So, my suggestion is to see how fast it responds to a rich air/fuel mixture. If it responds too slowly, the sensor needs to be replaced. For this test go to: TEST 2: Manually Creating A Rich Condition To Test The O2 Sensor.

CASE 2: The O2 sensor voltage was stuck above 0.5 Volts as the engine idled. This test result tells you that the O2 sensor is seeing a constant rich air/fuel mixture. This could be a result of an engine performance issue or the O2 sensor could be bad.

To find out, the next step is to create a lean air/fuel mixture to see if the O2 sensor reacts to it. For this test go to: TEST 3: Manually Creating A Lean Condition To Test The O2 Sensor.

CASE 3: The O2 sensor voltage was stuck below 0.5 Volts as the engine idled. This test result tells you that the O2 sensor is seeing a constant lean air/fuel mixture. This could be a result of an engine performance issue or the O2 sensor could be bad.

To find out, the next step is to create a rich air/fuel mixture to see if the O2 sensor reacts to it. For this test go to: TEST 2: Manually Creating A Rich Condition To Test The O2 Sensor.

TEST 2: Manually Creating A Rich Condition To Test The O2 Sensor

The two most common oxygen sensor failures are either the sensor not reacting fast enough to changes in the air/fuel mixture, or not reacting at all.

The cool thing is, you and I can easily check if this is happening by inducing a rich condition and seeing if the O2 sensor responds.

The fastest and easiest way to create a rich condition is by spraying a little starting fluid into the engine while it's running. My go-to method is spraying it into a vacuum hose.

Once the starting fluid hits the cylinders, you'll get an instant rich condition. The O2 sensor should then jump to its maximum voltage (0.900 Volts or higher), and you'll see it on your scan tool in Live Data mode.

Alright, this is what you'll need to do:

1. 1

   Loosen the top part of the air filter box.
   
   We're going to spray carb or starting fluid into the top portion of the air filter assembly so the engine can suck it in while it's running in the next steps.

2. 2

   Start your Honda and let it idle for about 5 minutes, since you need a warmed-up engine to get the O2 sensor to activate.

3. 3

   Connect your scan tool and go to its Live Data mode.

4. 4

   Once you're in Live Data mode, scroll down to the PID labeled O2S11.

5. 5

   You should see the O2 sensor voltage moving between 0.100 and 0.900 Volts constantly.
   
   If the voltage value stays fixed, don't worry yet —just continue to the next step.

6. 6

   With the engine running, spray a little starting fluid into the air intake duct through the top part of the air filter box while you watch your scan tool's display.
   
   NOTE: If you spray too much, the engine may stall. If it does, just restart the engine, repeat the step, and spray less fluid.

7. 7

   The oxygen sensor should immediately report 0.800 to 0.900 Volts as you spray short bursts of starting fluid into the air intake duct. As long as you're spraying (without stalling the engine), the voltage should stay there.

8. 8

   When you stop spraying, the O2 sensor values should drop and within a few seconds start oscillating again between 0.100 and 0.900 Volts.

OK, the test is done, let's take a look at what your test results mean:

CASE 1: The front O2 sensor's voltage spiked to 0.900 Volts when you sprayed carb spray into the vacuum hose. This tells you the oxygen sensor is working fine at this point in time. It doesn't need to be replaced, since whatever's making the PCM think it's bad is coming from somewhere else.

For more info on this, go to the section: Oxygen Sensor Codes Keep Coming Back.

CASE 2: The front O2 sensor's voltage DID NOT spike to 0.900 Volts when you sprayed carb spray into the vacuum hose. This confirms that your Civic's upstream (front) O2 sensor is no longer doing its job. Time to replace the oxygen sensor.

TEST 3: Manually Creating A Lean Condition To Test The O2 Sensor

In this section we'll induce a lean air/fuel mixture to see if the oxygen sensor reacts and reports it. We'll do this by disconnecting a large vacuum hose.

Creating a big intake manifold vacuum leak (with the engine running) will instantly make the air/fuel mix lean. When that happens, the oxygen sensor should drop to its minimum voltage —somewhere between 0.100 and 0.200 Volts. You'll be able to see this low voltage on your scan tool's display.

The hose we'll disconnect is the one that runs from the intake manifold to the vacuum brake booster.

IMPORTANT: Before you begin the test, disconnect the brake booster vacuum hose from the brake booster with the engine OFF. Think of it as a dry run. That way, when the engine's running during the test, you won't have to fight the hose (if it hasn't been pulled off in years, it can get pretty stubborn about coming off).

Alright, here are the test steps:

1. 1

   Crank and start your Civic's engine. Let it run for about 5 minutes.

2. 2

   Go into your scan tool's Live Data function and scroll down to the PID for the upstream O2 sensor.

3. 3

   Slightly disconnect the vacuum hose with the engine running while you watch the O2S11 PID on your scan tool.
   
   Don't pull the hose all the way off —the engine will stall. If it does, just restart it and this time disconnect the hose only a little.

4. 4

   As air enters through the brake booster vacuum hose, the O2S11 voltage should drop to about 0.100-0.200 Volts.
   
   As long as the vacuum leak is there, the voltage should stay in that 0.100-0.200 range.

5. 5

   Reconnect the brake booster hose and stop the vacuum leak.

6. 6

   The voltage should now start moving up and down between 0.100 and 0.900 Volts.

Let's take a look at what your test results mean:

CASE 1: The voltage dropped to 0.100 Volt when you opened the brake booster vacuum hose. This tells you the oxygen sensor is working fine since it can react to a lean air/fuel mixture and report it.

If the oxygen sensor passed TEST 1 and passed this test (TEST 2), you can be certain the sensor is working properly and doesn't need to be replaced.

If the computer is still flagging the upstream (front) O2 sensor as defective, see this section: Oxygen Sensor Codes Keep Coming Back.

CASE 2: The voltage DID NOT drop to 0.100 Volt when you opened the brake booster vacuum hose. This means the oxygen sensor is defective, since it can't react to a lean air/fuel mixture. Replacing the O2 sensor will fix the problem.

Oxygen Sensor Codes Keep Coming Back

There's a good chance that you have tested the O2 sensor on your Honda and it tested good or you replaced it, but it didn't solve the problem.

When this happens, it usually means that the problem (that's made us think the O2 sensor was bad) is being caused by something else. That something else is causing a rich or a lean condition that is causing the O2 sensor code.

Some of the things that can cause a rich condition are:

• A cylinder misfire caused by:
  • Bad spark plugs.
  • Bad spark plug wires.
  • Bad distributor cap.
• Clogged or leaking fuel injectors that are not atomizing the fuel correctly.
• Bad fuel pressure regulator for leaking fuel into each vacuum hose.

IGNITION SYSTEM CHECKS:

• How To Test The Igniter, Ignition Coil Accord, Civic, CRV, and Odyssey (at: easyautodiagnostics.com).

Some of the things that can cause a lean condition are:

• Vacuum leaks caused by:
  • Bad intake manifold gaskets.
  • Leaking or broken vacuum hoses.
  • Extremely clogged fuel filter.
  • Bad fuel pump that is not producing enough pressure or volume.

FUEL PUMP PRESSURE TEST:

• How To Test The Fuel Pump (1995-2000 1.6L Honda Civic).

My suggestion to you is to take a good look at some of the above components and see what condition they're in or if they need to be tested before replacing any of them.

More 1.6L Honda Civic Tutorials

I've written quite a few 1.6L 'how to' tutorials that may help you troubleshoot the issues on your Honda vehicle. You can find the complete list at:

• 1.6L Honda Civic Index Of Articles.

Here's a sample of the Honda 1.6L articles you'll find:

• How To Test The MAP Sensor (1.6L Honda Civic).
• How To Test The Throttle Position Sensor (1.6L Honda Civic).
• How To Test Engine Compression (1995-2000 1.6L Honda Civic).
• How To Test The Throttle Position Sensor (1995-2000 1.6L Honda Civic).
• How To Test The Igniter, Ignition Coil Accord, Civic, CRV, and Odyssey (at: easyautodiagnostics.com).