error code 51

I have a 1996 Jeep Grand Cherokee, after rebuilding the engine and test-driving it for approximately 5 miles, I received an error code 51. Are there any adjustments to timing that could be causing an incomplete burn (i.e. distributor setting) or is the pre catalytic converter oxygen sensor at fault?

I have a 1996 Jeep Grand Cherokee, after rebuilding the engine and test-driving it for approximately 5 miles, I received an error code 51. Are there any adjustments to timing that could be causing an incomplete burn (i.e. distributor setting) or is the pre catalytic converter oxygen sensor at fault?

51 is Air/Fuel at limit. oxygen sensorsignalinput indicates LEANair fuel mixture during engine operation. if you didn't change the oxygen sensor with engine change. change sensor TIMING IS CONTROLED BY COMPUTER. NON-ADJUSTABLE. DALE