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Last updateWed, 18 Oct 2017 10am

On The Road

Patching up a problem

A customer recently brought in his 2004 Honda Accord EX because the check-engine light went on. I asked him if the car was doing anything out of the ordinary, and he said no.

Once the car was in the shop, we connected the scanner and pulled codes. We pulled Code P0420 (catalytic converter system efficiency below threshold) and P1167 (air-fuel ratio sensor line voltage high).

Before I go further, let me explain that an air-fuel ratio sensor is an oxygen sensor, with a much larger bandwidth than a regular oxygen sensor. The air-fuel ratio sensor can measure 0-3 volts, while a standard oxygen sensor measures 0-1 volt. The air-fuel ratio sensor was created because modern cars can process more engine fuel information.

The catalytic converter efficiency is measured by looking at the front air-fuel ratio sensor and the rear oxygen sensor. The air-fuel ratio sensor sits in the exhaust manifold or in the pipe between the exhaust manifold and the catalytic converter. The rear oxygen sensor sits in the exhaust after the catalytic converter.

Each one of these sensors measures the amount of oxygen in the exhaust. If there is too much oxygen in the exhaust, the engine control module (ECM) will start to add or subtract fuel.

By controlling the amount of fuel, ECM performs the stoichiometric theory. Perfect stoichiometric is 14.7 parts of oxygen to 1 part of fuel.

I like to use the match example. If you were to strike a match, you will see the life of the flame. As the match starts to burn, it has its fuel and oxygen. As the flame has fully engaged the fuel, the flame burns perfectly. As the match runs out of fuel, the flame starts to flicker and smoke. There is excess oxygen, but not enough fuel. This is why oxygen sensors can give the ECM excellent engine efficiency data. This data on U.S. and Japanese cars is called “fuel trim”; on European cars, it’s known as “lambda.”

The ECM then takes the data from the front air-fuel ratio sensor and from the rear oxygen sensor and calculates the efficiency of the catalytic converter. The ECM can see if the catalytic converter is doing its job of filtering out hydrocarbons, carbon dioxide and nitrogen oxide.

Diagnosing the problem

We first checked how the front air-fuel ratio sensor was performing. The sensor had power and ground, and it was giving us a clean signal. At that point, we decided to check the rear oxygen sensor as well. This sensor also tested OK. We then made sure that the ECM was reading both the front and rear oxygen sensors correctly, which it was.

Next, we ran the test plan for the catalytic converter. While checking the car’s data, we saw an update to the technical information. It stated that if we got a P0420, check the ECM software level first. If our car did not have the latest software, we were supposed to update the engine control module (Tech Service Bulletin 03-063). We discovered that this car did not have the latest software update.

We have the Honda factory scanner and were able to update the software. We then performed an end-of-line test by taking a test-drive. Both the oxygen sensor and catalytic converter monitors passed the test.

Most cars built in the year 2000 and beyond have reflash, or software update capabilities. When a car manufacturer figures out that the software installed in the car has a bug and can’t run a system or a test properly, it often writes a software patch. This is why it is so important to follow the test plan and check for tech bulletins and software updates.

In the end, the software did the trick.

Twenty years ago, I rarely had to program a car, but today we do so weekly. It is interesting and exciting to see where the auto industry is going. I look forward to all of the advancements to come.

Matt Pataky owns Sunnyvale Foreign Car Service, 15 Pioneer Way, Mountain View. For more information, call 960-6988, email This email address is being protected from spambots. You need JavaScript enabled to view it. or visit sunnyvaleforeigncar.com.

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