Rick's Free Auto Repair Advice

What’s the best way to bypass an O2 sensor?

O2 spacers rarely work, but some simulators do

You can bypass an O2 sensor as long as we’re talking about the post-cat O2 sensor. When people ask this question it’s usually because they’re getting a P0420 or P0430 catalytic converter code and they want to pass an emission test without replacing the cat converter. To do that, they want the post-cat sensor to show a flat waveform to the ECM.

If your emission test includes a visual inspection and you install a spacer, you’ll automatically fail the test. So be warned!

Using a spacer to bypass an O2 sensor

An O2 spacer puts the O2 sensor farther away from the exhaust stream, causing it to miss some of the lean and rich exhaust pulses.  If the catalytic converter occasionally sets a P0420 trouble code, a spacer might work by smoothing out the waveform coming from the post cat O2 sensor. However, if your emissions testing facility performs a visual inspection, you’re gonna get caught and fail.

O2 spacer






02 simulator

When using an O2 spacer, the spacer screws into the cat/exhaust downstream bung and the sensor screws into the spacer. So the sensor only sees a small portion of the exhaust

Using an O2 simulator

Many companies call their spacers a simulator; a spacer is just that, a spacer. A real simulator is either an electronic device that reinterprets the O2 sensor signal and provides a flat line signal to the ECM, OR it contains a small catalytic converter to clean up the small amount of exhaust that reaches the real O2 sensor.

02 simulator mini catThe “mini-cat” shown here is an O2 spacer with a small insert of catalytic material to treat the exhaust reaching the O2 sensor. The treated exhaust then allows the O2 sensor to report the correct values to the ECM. It’s not really a good way to bypass an O2 sensor.

Downsides to the O2 spacer and mini-cat simulator

Neither the spacer nor the mini-cat are long term solutions to a P0420 or P0430 problem. They can only be used to try and fake out the smog test. Here’s why:

How the ECM tests the catalytic converter

In normal operation the ECM is constantly adjusting the air/fuel mixture to obtain the right amount of power with the lowest emissions. The ECM reads the upstream O2 sensor and makes adjustments if the exhaust is too rich or too lean. So the upstream O2 sensor data is constantly changing. However, if the catalytic converter is working properly, stores oxygen and uses that oxygen to help burn off excess hydrocarbons like fuel and oil. So the downstream post-cat sensor should be reading almost a flat line; neither rich nor lean. When a catalytic converter fails, the downstream O2 sensor follows the upstream waveform because the cat converter can absorb oxygen or burn hydrocarbons.

To test the efficiency of the catalytic converter, the ECM purposely commands a quick very rich mixture to fill the cat with excess fuel, followed by a quick lean mixture to fill it with oxygen. Because these are rapid changes, the catalytic converter can’t react fast enough to remove the excess hydrocarbons or absorb all the extra oxygen. During the test the ECM expects the downstream sensor to see these extremes and report them to the ECM. Then, the ECM goes back to normal air/fuel mixtures and it expects the cat converter to respond by cleaning up the exhaust and the post cat O2 should report a flat line again.

If the post cat sensor doesn’t fluctuate as expected during the test or flat line after the test, the ECM will set a P0420 or P0430.

That’s why O2 spacers and O2 mini-cat simulators don’t work over the long term: they fail the cat test.

Actual O2 simulators can handle the test by expecting the test and altering the results

I’m not endorsing this deceptive practice, I’m only explaining how it works. Here’s a description from EZ CEL:

“EZ CEL Fix oxygen sensor eliminator gives the ECM with mimicked, fluctuating voltage signal and tricks the Power Control Module into thinking the emissions are within allowance. Car Truck SUV oxygen sensor simulator will allow you to drive without converter and still keep the ECU warning light off. A popular alteration is to decat the exhaust or install an aftermarket header, to free up the performance level. Exhaust gas is controlled through 2 to up to 4 oxygen sensors. The front sensor controls the fuel management system and aftercat monitors converter function. When headers are installed or catalysts removed, the result is a catalyst efficiency check engine light failure code alerting the Powertrain Control Module that converter is missing or not working properly. Modified engine produces a larger amount of exhaust gases than allowed. You do not want to drive in safe mode with constant MIL light, and your Car Truck SUV will not pass emission test in many US states and countries. The ultimate solution is to wire up O2 sensor simulator to the lambda probe’s harness to mimic the o2 sensor.”—https://www.magnumtuning.com



Simulators have to be wired into the wiring harness to the post cat O2 sensor. And they have to be hidden if you have visual inspections.

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