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Hi Rhino Volvo,
What year 240 is yours? I'll guess at least a 1989 and newer? How many miles? How old is the coolant? Have you performed any work on the engine cooling system (radiator, engine coolant replacement (antifreeze + distilled [never tap] water, thermostat, hoses).
You searched for and replied to a thread dated from year 2008.
You have checked the OBD-1 socket 2 (fuel injection and connected emissions sensors and devices). Have you checked exhaustively any other codes in socket 2 and also socket 6 (ignition and connected emissions sensors and devices).
See:
Engine and OBD Diagnostic Codes - https://www.brickboard.com/FAQ/700-900/EngineOBDCodes.htm
Codes for LH-Jetronic (fuel) 2.4 / 3.1, EZK (ignition) 116.
Code 1-2-3 is an engine coolant sensor fault. The on-dash orange Check Engine indicator illuminates.
The ECT sensor is two engine coolant sensors in one: One sensor for the fuel injection ECU and the other for the ignition ECU. So, checking socket 6 OBD may help with the diagnosis. Repeat OBD check in the event the ECUs in your model year Volvo 240 have other fault codes stored.
The fault may be the ECT sensor itself or the engine control wire harness between the ECT and the engine control units (ECU). Includes all connectors at the ECT and the ECU and in between.
So, with your trusty voltmeter, you can test the sensor wiring. This board is replete with the procedure in various forms to do so. May be nothing more than, with battery disconnected, a disconnect, reconnect on the wire harness connectors, or may involve ECT replacement. ECT replacement is easier, or best, with the intake manifold removed. (Some are able to replace the ECT without air intake manifold removal. As seen on a You Tube video or two with a tool set-up.)
See line art wiring diagrams here for your model year Volvo 240:
http://www.volvowiringdiagrams.com/?dir=volvo/240%20Wiring%20Diagrams
The ECT sensor is somewhat difficult to reach. You may need remove the air induction piping under the throttle body to get at the connector. And even then it will be difficult. Small hands help.
Or, you can, at the ECU connector, back probe the specific pin positions and perform a continuity and resistance check.
Other members can provide you guidance as to diagnosis using fewer words.
You are in TN-state? Have much rust / corrosion about the engine bay?
Hope that helps get the help to you started.
Earl Grey Tea Boy with hunny and milk.
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