Impco HHI, Service Manual

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DTC 1156-Closed Loop Low (Bi-Fuel/Gasoline Only)
Conditions for Setting the DTC

Heated Oxygen Sensor

Check Condition-Engine Running

Fault Condition-Closed loop multiplier out of range greater than-35%

MIL-On
Circuit Description
The EGO 1 sensor is used to determine if the fuel flow to the engine is correct by measuring
the oxygen content in the exhaust gas. The ECM uses this information to correct the fuel flow
with the Closed Loop multiplier and Adaptive multiplier. This fault will set if the adaptive multip-
lier exceeds the limits of normal operation. Always run the fuel system diagnostic checks
before using the following diagnostic chart.
Diagnostic Aid
Fuel System The system will be rich if an injector fails to close properly. High fuel pressure
due to a faulty fuel regulator or obstructed fuel return line will cause the system to run rich.
Ignition noise Open or poor ground circuit to or in the ignition system or ECM wiring may
cause EMI (Electromagnetic interference). This noise could be interpreted by the ECM as igni-
tion pulses or voltage across the oxygen sensor input causing the system to run rich.
MAP Sensor A higher manifold pressure than normal can cause the system to run rich. Tem-
porarily disconnecting the MAP Sensor will allow the ECM to set a default value for MAP and is
a good way to quickly eliminate this as a problem.
IAT Sensor Check for a shifted sensor that could cause the ECM to sense lower than actual
temperature of incoming air. This can cause a rich exhaust condition.
ECT Sensor Check for a “skewed” sensor that could cause the ECM to sense engine tempera-
ture colder than it actually is. This can also cause a rich exhaust condition.