Cylinder Power Imbalance Detected Page 1 of 5
FAULT CODE 951
Cylinder Power Imbalance Detected
Overview
CODE REASON EFFECT
Fault Code: 951 Cylinder power imbalance between cylinders. A Possible low power,
PID: P166 power imbalance between cylinders was rough idle, or
SPN: 166 detected by the ECM. misfire.
FMI: 2
LAMP: None
SRT:
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:
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z Faulty engine brakes (look for brakes out of adjustment or oil supplied to brake
actuator when brakes are off)
z Valve lash out of adjustment (compare actual valve lash to the reset limits)
z Cylinder failure (check for high blowby when the engine is loaded)
z Valve failure or valve held open (listen for intake valve “chirping” noise in the intake)
z Cylinder head or gasket failure (look for signs of oil, coolant, or casting flash in the
intake or exhaust passages)
z Extreme camshaft wear (inspect the camshaft lobes for wear)
z Extra, missing, or damaged sealing washer under an injector
z 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
z Injector failures (“swap” injectors to see if the problem “follows” the indicated injector).
Troubleshooting Steps
STEPS SPECIFICATIONS
STEP 1. Check for multiple fuel system
fault codes.
STEP 1A. Read the fault codes. Fuel system component fault codes
active or have accumulated inactive
counts?
STEP 2. Perform diagnostic tests.
STEP 2A. Perform the cylinder All cylinders test okay?
cutout test and/or the automated
cylinder balance diagnostic test.
STEP 3. Clear the fault codes.
STEP 3A. Disable the fault code. Fault Code 951 inactive?
STEP 3B. Clear the inactive fault All fault codes cleared?
codes.
Guided Step 1 - Check for multiple fuel system fault
codes.
Guided Step 1A - Read the fault codes.
Conditions
z Connect INSITE™ electronic
service tool.
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Cylinder Power Imbalance Detected Page 3 of 5
z Turn keyswitch ON.
Action
Read the fault codes.
z Start the engine and let it idle for
one minute.
z Use INSITE™ electronic service
tool read the fault codes.
Fuel system component fault codes Fuel system component fault codes
active or have accumulated inactive active or have accumulated inactive
counts? counts?
Troubleshoot active fault codes first.
YES NO
Troubleshoot full system component fault No Repair
codes.
Appropriate fault code troubleshooting
Go to 2A
tree
Guided Step 2 - Perform diagnostic tests.
Guided Step 2A - Perform the cylinder cutout test and/or the automated
cylinder balance diagnostic test.
Conditions
z Turn keyswitch ON.
z Allow the engine to run at idle
speed.
z Connect INSITE™ electronic
service tool.
Action
Use the automated cylinder balance
diagnostic test to determine the power
contribution of each cylinder.
All cylinders test okay? All cylinders test okay?
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Cylinder Power Imbalance Detected Page 4 of 5
YES NO
No Repair Failure of cylinder test can be due to faulty
engine brakes, valve lash out of
adjustment, cylinder failure, valve failure,
cylinder head or gasket failure, extreme
camshaft wear, or injector failure.
Investigate, diagnose, and correct the
condition of the applicable component that
is possibly out of adjustment or
malfunctioning. “Swap” injectors between
cylinders to see whether or not the
problem follows that injector.
Troubleshooting and Repair Manual,
Go to 3A ISBe, ISB, and QSB5.9 (Common Rail
Fuel System), Bulletin 4021271
Guided Step 3 - Clear the fault codes.
Guided Step 3A - Disable the fault code.
Conditions
z Connect all components.
z Connect INSITE™ electronic
service tool.
z Turn keyswitch ON.
Action
Disable the fault code.
z Start the engine and let it idle for
one minute.
z Verify that the fault code is
inactive.
Fault Code 951 inactive? Fault Code 951 inactive?
YES NO
No Repair No Repair
Go to 3B Go to 1A
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Cylinder Power Imbalance Detected Page 5 of 5
Guided Step 3B - Clear the inactive fault codes.
Conditions
z Connect all components.
z Connect INSITE™ electronic
service tool.
z Turn keyswitch ON.
Action
Clear the inactive fault codes.
z Use INSITE™ electronic service
tool to erase the inactive fault
codes.
All fault codes cleared? All fault codes cleared?
YES NO
No Repair No Repair
Repair complete Appropriate troubleshooting steps
Last Modified: 15-Mar-2005
Copyright© 2005
Cummins, Inc.
All rights reserved
11:11:31 2007-07-20
