I just changed out the O2 sensor, and the car was running great, until it just threw the check engine light again. I checked on socket 2 and got 5-1-1.

What in the heck is 5-1-1???? I've reset the light, and will drive around again later to see if it comes back. Is this a fluke? Thanks

Did you replace with a Volvo OE sensor or one that you had to swap out the connectors?

DEWFPO
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1998 S90 070,050 and 1995 964 153,010

I used a Mustang sensor. The signal voltage should be same for all sensors, but heater wattage might be different? Could this be an issue?

If you soldered the old connector on to the Mustang sensor, you must make sure the connections are as perfect as you can get them. The sensor generates only 0.2-1.0 volts so any resistance in the solder connections will affect the voltage the ECM sees.

DEWFPO
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1998 S90 070,050 and 1995 964 153,010

511 - Long term fuel trim ( idling) if it is a 960 and has Motronic 1.8.

Air leakage, defective MAF sensor, low/high fuel pressure, faulty sensor signals, leaking injectors, leaking EVAP system, contaminated engine oil, oil level too high. Are usually the fault sources by the book I have.


DanR 94 964 262,000 miles (28,000 on the new engine)
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DanR

Oy... that's a lot of possible causes.

The engine takes quite a few rotations at startup before it will catch... I've assumed this to be a defective fuel line check valve, and have been meaning to replace it. Could this lead to low fuel pressure?

Does anyone have experience with this code? Idle seems to be fine... perhaps the slightest hesitation just off idle, when I tip into the gas.

I'll do some more research. Thanks Dan