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240 Oxygen Sensor Check engine problems

rileybaldone14

New member
Joined
Apr 8, 2019
Location
California
Hi

I have an 89 244 DL, recently the check engine light has been coming of with a 2-1-2 code, which bluntly reads "oxygen sensor" in the books. Thing is, the light only comes on when the vehicle is allowed to idle. When driving, the light stays off indefinitely. When allowed to idle for only a few minutes, the light comes right back on. I've replaced the sensor, checked the wiring, and not had any luck.
The car runs great, I have no complaints whatsoever aside from this pesky light.

Id greatly appreciate any help, as this problem has been getting increasingly irritating to try and solve.
 
Have you confirmed power and ground to the heater circuit? A blown fuse is common.

^^
This is usually the answer to 212 at idle. The heater power comes from fuse 4, and rather than being blown, it is often just not making good connection.

To double check, have your DMM ready to measure resistance, then turn the car off, disconnect the heater connector, and quickly measure toward the oxygen sensor's two white wires. A working heater will show the heater cooling from about 13 ohms hot to about 4 ohms cold, in about one minute.

If you're indeed confident the heater is working, then monitor the sensor's output voltage to determine what the air/fuel mixture might be at idle.
 
Unfortunately im short on time today so i will check everything tomorrow, sensor heater power, fuses etc and report here what happens. Thanks
 
Apparently solved, corroded fuse is to blame. I measured voltage on fuse 4 and saw nothing, voltage on fuse 4's socket would jump between 0 and 12. I took fuse 4 out, scrubbed it a bit and measured again, consistent 12.5 across the fuse and the light has stayed off.
Thank you ZVOLV and cleanflametrap a bunch for your help.
 
Nice. Always perform circuit verification before replacing the component. 2 minutes with a meter between the two white wires for the heater power and ground would have easily revealed the issue. I will admit I have replaced parts based on codes alone and skipped the circuit verification, but that's a very sloppy practice.

Glad you got it sorted!
 
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