CM850 Electronic Control System – FAULT CODE 951

Cylinder Power Imbalance Detected


Fault Code: 951
PID: P166
SPN: 166
FMI: 2
LAMP: None

Cylinder power imbalance between cylinders. A power imbalance between cylinders was detected by the ECM.

Possible low power, rough idle, or misfire.

Cylinder Circuit

Circuit Description

An algorithm in the electronic control module (ECM) calibration monitors engine speed as each injector fires while the engine is at idle. If a cylinder has high or low contribution to engine speed, this fault will become active. The intention of this fault is to assist in troubleshooting performance complaints such as engine misfire and low power. A fault lamp does not illuminate while this fault code is active.

Component Location

The ECM is located on the intake side of the engine, below the air horn. Refer to Procedure 100-002 for a detailed component location view.

Shop Talk

If Fault Code 951 is inactive in the ECM memory and there is no complaint such as “low power” or “engine misfire,” it is possible that an intermittent event such as air entering the fuel system following a filter change caused the fault. Inactive counts of Fault Code 951 should be disregarded unless other performance symptoms are present.

Causes of cylinder imbalance can include:

  • Faulty engine brakes (look for brakes out of adjustment or oil supplied to brake actuator when brakes are off)
  • Valve lash out of adjustment (compare actual valve lash to the reset limits)
  • Cylinder failure (check for high blowby when the engine is loaded)
  • Valve failure or valve held open (listen for intake valve “chirping” noise in the intake)
  • Cylinder head or gasket failure (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
  • Extremely damaged HPC/injector interface (examine the high-pressure fuel connector at the injector end); this problem will also cause excessive fuel return and Fault Code 2292/2216
  • Injector failures (“swap” injectors to see if the problem “follows” the indicated injector).

Refer to Troubleshooting Fault Code t05-951

Last Modified:  08-Jul-2010