My 1995 XJ6L Check Engine light just came on. When I checked the DTC, it displays "P1178" which has to do with O2 Sensor/electrical/fuel issues.
All the cats were replaced in 2018, low mileage since then and she past smog test 2 months ago.
Any suggestions as to where to start diagnosing is appreciated.
Thanks!

 

The P1178 code relates to the Long Term Fuel Trim being too lean, suggesting an air flow issue. Maybe a vacuum leak somewhere on the intake..
I would check and make sure all of the ail of the Air Intake hoses and vacuum lines are good and secured, It would also be worth making sure the exhaust downpipe manifold connections to the cats are properly bolted in.
The Long and Short Term Fuel Trims are most likely to be well below zero, This would be worth monitoring while you are checking everything over

 

Thanks for that info! Gives me a place to start.