Volvo Repair: Check engine light on, showing no codes, volvo 240 dl, emission control system

Question
Hello.  I got my 1989 Volvo 240 DL over the summer from my aunt.  It has 344K miles on it and still looks and runs considerably well.  She always took it for servicing to the Volvo dealer.  Lately it has been sluggish on acceleration.  It recently failed emissions due to high HC.  I replaced the plugs, wires, cap, rotor, and air filter.  It passed emissions on the second attempt, however it is still sluggish.  I tested the oxygen sensor.  It seems to have tested okay, registering voltage in the high range upon warm-up, seeming to indicate that it is indeed running rich.  Next, I attempted to check for codes, even though the check engine light had not come on.  I erroneously connected the test lead to terminal 2 first (this terminal is for older cars).  Realizing my error, I then connected it to terminal 6.  I got a code 2-1-4 (speed sender signal).  Figuring that this may have been an old code not yet cleared, I cleared the memory.  Now the check engine light stays on, but registers no codes!  The car still runs sluggishly.  Also, This autumn, the radiator started leaking.  While nursing the car to the weekend so I could fix it, I kept a close eye on the coolant temperature gauge.  I noticed that it was slow to register the coolant temperature and seemed to be somewhat intermittent, showing no temperature at times.  Anyway, I replaced the radiator and the gauge still behaves similarly. Sometimes when the engine is cold I notice that the sluggishness has all but disappeared.  This makes me wonder if the coolant sensor is faulty (is this temperature gauge sender even integral to the emission control system, or is there an additional sender?).  I have yet to check the resistance of this sensor (it looks kind of difficult to access).  If you have any ideas how to get the check engine light to go out, or how to get the engine running better, I'd be appreciative.  Thank you.


Answer
A faulty ECT could cause that.  The computer uses the ECT to determine if its in warm up mode, and then richens the mixture to heat it up faster.  So it could be keeping the mixture overly rich because it still thinks the engine is cold.  As for the engine light, have you checked other systems that could have posted codes?  Any system that may have an affect on emissions will turn on the check engine light.  Double check every one.  If it still stays on, try disconnecting the battery, touching the cables together and then reconnect the battery.  That should power down the computer and maybe reset it so the light goes off if there is infact no codes.  If it still remains on without any codes then you have a failure in the light circuit that is keeping it on.  Shouldn't be too hard to find since it has to be a short to keep it on.  Good luck.

James