Pin down the cause of alerts

Q: My questions are related to the oxygen sensors that have triggered the "check engine" light on my 1996 Ford Taurus wagon GL. I recently purchased this car. It has 76,000 miles on the odometer and had been parked for more than two years due to a mechanical problem.

When I started the engine for the first time, the gasoline in the tank was more than two years old and the tank was less than one-quarter full. The engine started quickly and ran smoothly. Everything seemed to be OK, and the "check engine" light wasn't on.

I added 4 ounces of injector cleaner and drove the car. Within two miles, the "check engine" light came on. Afterward, the car ran for 15 more miles on the old gasoline.

I had a scanner connected and it showed codes P0135, P0141, P0155 and P0161 - oxygen sensor heater problems.

Could the old gasoline have contaminated or damaged the four sensors in such a short of time? If I keep driving with defective sensors, will this damage the catalytic converter? Could driving using good gas help restore the sensors? If not, will the "check engine" light clear after the oxygen sensors are replaced, or do I need a scanner to clear it?

A: So many good questions and so little space to answer them.

Your Taurus uses four oxygen sensors, one before and after its two catalytic converters, for each of the V-6 engine's two banks of cylinders.

The two front oxygen sensors are used by the engine computer to monitor exhaust content and fine-tune fuel injection for best engine operation.

The two rear sensors are used to validate catalytic converter function.

A heater is built into each sensor to quickly bring them up to a minimum operating temperature of about 600 degrees F at each startup and keep them hot as the engine idles.

While the sensor portion of an oxygen sensor can be readily damaged by contaminants such as lead, coolant, silicon sealers and oil burning, the heater portion would not be affected. So stale gas is unlikely to be a concern.

Oxygen sensor heaters aren't the most bulletproof components, but it would be odd to encounter four bad ones at the same time.

Each heater is individually controlled or grounded by the engine computer - it's unlikely to have four faults there.

But they are all powered by fuse 27 in the under-hood fuse/relay box. Be sure to check this.

A wiring problem is also possible - perhaps the red wire feeding these components has broken, preventing heater operation, or shorted, blowing the fuse.

Testing the circuit isn't especially difficult, following applicable service manual procedures.

It's OK to drive the car while you're sorting this out. The oxygen sensors will function normally once they belatedly warm up, except you'll be unaware of additional faults because the "check engine" light is already on.

Check with the scanner to be sure.

Once the fault is rectified, the "check engine" light will go off by itself after several driving sessions, and the codes will erase after 40 run cycles.

You can also clear the light and codes immediately, using a scan tool.

Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose, Calif. Readers may send him e-mail at under-the-hood@earthlink.net; he cannot make personal replies.