How do you read codes for 1991 Volvo 740?

To read codes for a 1991 Volvo 740, you need to locate the diagnostic connector, typically found near the driver's side under the dashboard. Use a jumper wire to connect terminals A and B in the connector. Then, turn the ignition key to the "on" position without starting the engine; the check engine light will flash a series of codes that correspond to specific fault codes. Count the flashes to identify the codes, which can be referenced in the service manual for diagnosis.