Hi all,

I have been experiencing an intermittent problem with proper air/fuel mixture on my 1991 240. I had it serviced about 4 months ago at a Volvo dealer who checked for possible air leak and could not find any. The O2 sensor was fine. The fault codes at that time were 231 and 221 (which tripped the check engine light). The technician reset the codes and all has been well until the other day when the check engine light came on showing the same fault codes as before.
The only difference now is that even though I erased the fault codes, the check engine light immediately comes on after starting the engine showing the above fault codes(in the past it took a while before it came on).

My question is that barring any leaks, could the problem stem from the O2 sensor even though the fault code for the sensor is not displayed? Also, could the problem be related to a possible faulty thermostat within the preheat air system as I noticed that hot air comes in from the vents for about the first 15 to 20 minutes even though the heater valve under the dash is completely closed? Is there anything else that could be causing this problem?

If I do have to replace the O2 sensor is this something that is relatively easy to do for a lay person?

Thank you for your help.