SPN 166 - FMI 2 (Fault Code 951)

Cylinder Power Imbalance Between Cylinders - Data Erratic, Intermittent, or Incorrect

Engine Control Module (ECM)

Fault Code: 951
PID(P), SID(S): P166
SPN: 166
FMI: 2/2
Lamp: None

REASON: A power imbalance between cylinders was detected by the ECM.

EFFECT: Engine will misfire or run rough.

Circuit Description:
An algorithm in the ECM calibration monitors engine speed as each injector fires. If a cylinder(s) has high or low contribution to engine speed, the fault will become active. The intention of the fault is to assist in troubleshooting performance complaints such as engine misfire and low power.

Component Location:
The Engine Control Module (ECM) is mounted on the intake manifold side of the engine.

Conditions for Running the Diagnostics:
This diagnostic runs continuously when the keyswitch is in the ON position or when the engine is running.

Conditions for Setting the Fault Codes:
The Engine Control Module (ECM) detected a power imbalance between cylinders greater than the pre-defined limits.

Action Taken When the Fault Code is Active:
• The ECM illuminates the amber CHECK ENGINE lamp immediately when the diagnostic runs and fails.

Conditions for Clearing the Fault Code:
• To validate the repair, perform a key cycle, start the engine and let it idle for 1 minute.
• The fault code status displayed by the recommended Cummins® electronic service tool or equivalent will change to INACTIVE immediately after the diagnostic runs and passes.
• The ECM will turn off the amber CHECK ENGINE lamp immediately after the diagnostic runs and passes.
• The “Reset All Faults” command in the recommended Cummins® electronic service tool or equivalent can be used to clear active and inactive faults.

Shop Talk:
-Causes of cylinder imbalance can include:
-Valve lash out of specification (compare actual valve lash to reset limits)
-Cylinder damaged (check for high blowby when the engine is loaded)
-Valve damaged or valve held open (listen for intake valve "chirping" noise in the intake).
-Cylinder head or gasket damaged (look for signs of oil, coolant, or casting flash in the intake or exhaust passages)
-Extreme camshaft wear (inspect the camshaft lobes for wear)
-Extra, missing, or damaged sealing washer under an injector

Read More:
SPN 1590 - FMI 2 (Fault Code 784)
SPN 1349 - FMI 7 (Fault Code 758)


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