Your 1990 Volvo 240 comes equipped with the OBD-1 diagnostics for the Bosch 2.4 LH fuel injection and the Bosch EZK (and EGR, if equipped, through the EZK) ignition systems control systems.

While not conclusive, you could use the OBD-1 as a start for fault diagnostic in seeking the cause or causes as to poor running and fuel economy that may lead to premature catalytic convertor failure.

As others have said, high NOx, can be caused by:
- Lean running condition (Air intake leak, spark plugs too hot, faulty coolant temp sensor, )
- Exhaust system leak ahead, usually high up at the manifold, of the catalytic convertor with weak or failed 02 sensor
- Overheated AMM (or MAF) as the air box temp control flap valve is stuck on hot air in only if you have the silver accordion hose connected to the exhaust preheater plate and the heated air intake on the air filter box.
- Faulty timing (I forget whether too advanced or to late)

It may be very possible you may have a rich running condition (excess fuel) and some conditions causing excessively hot combustion chamber or exhaust temperatures.

Please see (http://www.brickboard.com/FAQ/700-900/EngineOBDCodes.htm).

If not already, please acquire a blue Bentley Volvo 240 service manual.

Hope that helps.

Not Loggedin MacDuff
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Fuzzy kitties.