Twelve years and more than 100,000 miles have passed under your trusty commuter and the Check Engine light has never, ever, winked at you … until yesterday, when it coincidentally anticipated your state inspection appointment at the end of the month. Rats! The car will never pass the emissions test with that light on. Now what?

Here’s the perfect opportunity to break out that new, easy-to-use, consumer-grade OBD II (On-Board Diagnostics II) generic code reader. That’s what we did on our sacrificial lamb, a Nissan Altima. After plugging in the universal connector under the dash, we retrieved a code P0136 “O2 Sensor Circuit Malfunction (Sensor 2).” This let us zero in on the likely problem right away. Remember, a trouble code stored in the engine computer doesn’t necessarily tell you what’s wrong. It’s just a good starting point.

So, it’s time to actually check out the sensor. Sensor 2 is the downstream sensor, in the catalytic converter, smack in the middle of the underside of the car. Start by getting the car up on some safety stands, then roll underneath it with a creeper.

Lazy O₂ Sensor Equals Low MPG

OBD I engine management, dating back to 1980, used just one upstream O2 sensor, mounted in the exhaust manifold, as close as possible to the cylinder head’s heat. That was because an oxygen sensor can’t produce and send the rapidly toggling voltage signal the engine computer is expecting until the sensor is really hot (above 600 F). That’s why today all O2 sensors are electrically heated — so they will start working sooner.

It’s always the upstream sensor (“Bank 1, Sensor 1”) that the powertrain management system pays attention to in order to fine-tune the proper 14.7:1 air/fuel ratio (aka lambda). Lambda is the Greek character used to designate that perfect stoichiometric ratio. And although the modern oxygen sensor has a 100,000-mile life expectancy, when it gets old and lazy you’ll begin to notice a drop in fuel economy. More extreme cases of malfunction will lead to driveability issues and, eventually, to an illuminated Check Engine light when the frequency of the sensor’s signal slows to a crawl.

Rules of Thumb

It is possible to do some simple checks on O2 sensors with a high-impedance digital voltohmmeter. You don’t necessarily need a professional technician’s scan tool. But to perform specific test procedures, it definitely helps to have the service manual for your vehicle. Some manuals, for example, give simple static resistance measurements across the sensor’s terminals. These alone may not be conclusive.

Do a dynamic test. Determine the frequency at which a good sensor is supposed to toggle back and forth from 0 to 1 volt while the engine is being revved. A general rule of thumb says the sensor should toggle two to three times per second at 2500 rpm. There are variations: ­Nissan says the rear heated oxygen sensor we replaced should read above 0.6 volt at least once while racing the engine up to 4000 rpm (under no load). Your service ­manual will have the specifics for your vehicle.

Or, you can just replace your O2 sensor(s) at regular intervals, to try and prevent that minor drop in mpg sometimes caused by a “lazy” high-mileage sensor. Continued...

Simple Replacement

Our problem was obvious once we got under the car. The sensor’s harness had snagged on some road debris or, more likely, high-centered on a pile of ice here in New York last winter. With the harness dangling in the breeze, not connected to anything, it’s no wonder the Check Engine light was on.

But instead of just fixing the harness, we chose to go with a new $100 sensor. Its fresh electrical connector brought peace of mind. Besides, the old unit had been in service for 104,000 miles, so it was due for replacement. Despite the rust caked around the sensor’s mount, a good yank with an open-end wrench was all it took to break it loose.

We got lucky, because this O2 sensor was easy to get at. More often than not, you’ll need to acquire one of the specially slotted oxygen sensor sockets, available wherever mechanic’s tools are sold. The slot in the side of these sockets enables easy ­removal and installation, even in cramped engine compartments, and protects the sensor’s wire leads.

Finally, after the sensor fix and a certain amount of driving and key cycling, we were waiting for the Altima’s Check Engine light to go out by itself. It didn’t. So we had to diagnose a little further. At last, we found two weak engine grounds. Their terminals had oxidized slightly on the aluminum intake manifold and were causing enough of a voltage drop in the ground side of the sensor circuit to keep the engine computer on guard. This caused two additional codes to store in the system along with the O2 sensor code. Ultimately, loosening and retightening the two ground strap bolts allowed us to clear all the codes from ­memory. The Check Engine light blinked off and the car passed its emissions test with flying colors.

1] Sometimes you get lucky: The old sensor unscrewed easily.

2] A sparing dollop of anti­seize will make the next removal easier. New sensors already have some on the threads.

3] Thread the new harness back into the body of the car. Make sure the grommet seals out road dirt and water. Use cable ties to keep the harness tucked out of the way.