I have had a lot of problems with my 240 dealing with the check engine light. But after much pain I have determined the easiest solution is to start replacing parts. When I give up and take it to the pro mechanics that is what they do everytime. So here is what I would do.

1) Disconnect the negative ground cable to the battery for 5 minutes to reset the computer. Also if you have a Haynes or other manual follow their steps for resetting the computer. Then take the car for a test drive and see if the check engine light comes back on. You need to this because the computer can turn on the check engine light from a temporary situation. If it comes back on- continue through next steps.

2) I know you just did a tuneup 6 months ago, but do it again. I recently had your problem (check and see if my previous post is still in Opinions). It turned out that I had bought poor quality replacement parts. So change your spark plug wires, spark plugs, distributor cap/rotor with OEM parts- Bougicord spark plug wires, Bosch on rest. I have found out the hard way that using non-OEM causes problems especially with the spark plug wires.

3) Clean your throttle body and flame trap.

4) You may be able to carefully clean up your O2 sensor by removing and blowing out with compressed air. But if it has more than 30-50k miles, replace it. You probably have already damaged it.

5) I would bet the above will solve your problem. Although it will involve replacing parts ($$), but it will save you from the big repair bill from your friendly mechanic.

Oxygen Sensor Diagnosis. [Question: How do I diagnose oxygen sensor problems?] This is from Bosch information:

Test for a rich mixture as follows:

Disconnect the sensor lead to the control unit.

Run the engine at 2500 rpm

Artificially enrich the fuel mixture on electronic fuel injected engines by removing and plugging the vacuum line to the fuel pressure regulator.

If the voltmeter rapidly reads .9 volts, then the oxygen sensor is correctly sensing a rich mixture. But, if the voltmeter responds sluggishly, or if it stays below .9 volts, try running it at 3,000 rpm for a few minutes, then check again. No improvement means you buy a new sensor.

Test for a lean mixture as follows:

Induce a small vacuum leak

If the voltmeter rapidly drops to .2 volts or below in less than a second, then the oxygen sensor is correctly measuring the lean mixture. But if the voltmeter responds sluggishly, of if it stays above .2 volts, give it the 3,000 rpm treatment and try again. If no improvement, then the sensor should be replaced.

Test dynamic performance as follows:

Reconnect the sensor lead and tap your meter into the signal wire

Set the mixture to specification.

Run the engine at 1500 rpm.

You should see rapidly changing readings that average somewhere around .5 volt as the computer keeps adjusting the blend. The sensor output should fluctuate around .5 volts. If it doesn't, replace the sensor. Deciding whether or not response is slow enough to justify replacement requires some judgment. A common rule of thumb for minimum activity is eight trips across the rich/lean line in ten seconds, and sometimes you can find specs for cross-counts.

Yeah, you're getting the rich or lean at speed or something to that effect.

I've gotten those codes before, but ultimately fixed them by cleaning up the connection to the O2 sensor wire on the firewall, then resetting the codes. The same thing goes for the mass airflow sensor, which will throw a 1-2-1, I think. I cleaned that one up yesterday and reset it. We'll see what happens.

You can test the O2 sensor function, but first try cleaning and resetting the connection and find out what happens there. If it writes a code again, then go farther.

Joe:

check out the universal O2 sensors offered by FCP.com, RPR and others on Ebay. these go for about $90. Good luck, Genaro.