Diagnose and fix P1128 Chrysler, Dodge, Jeep
A P1128 Closed Loop Fueling Not Achieved – Bank 1 means that the PCM isn’t seeing the O2 readings it needs to change from open loop (default factory programming determines air/fuel mixture) to closed loop (PCM varies air/fuel mixture based on live readings from all sensors). If order to set a P1128 trouble code, the PCM must see that the O2 sensor on bank 1 hasn’t met the conditions to go into closed loop at least one time on each of two consecutive trips. If the PCM sees the data it needs to go into closed loop on three consecutive trips, it will clear the code and turn off the check engine light.
Possible causes for P1128
Here’s what the shop manuals says about possible causes:
RESTRICTED FUEL SUPPLY LINE
FUEL PUMP INLET STRAINER PLUGGED
FUEL PUMP MODULE
O2 SIGNAL CIRCUIT
O2 RETURN CIRCUIT
O2 SENSOR HEATER OPERATION
MAP SENSOR OPERATION
ECT SENSOR OPERATION
ENGINE MECHANICAL PROBLEM
FUEL FILTER/PRESSURE REGULATOR
POWERTRAIN CONTROL MODULE (PCM)
But what’s the most common cause for a P1128?
You can check all those components. But the most common cause is a bad O2 sensor. You may see live data for the bank 1 O2 sensor that meets the parameters, but don’t be fooled by that. This is one case where you shouldn’t trust live data and instead slap a DSO right on the O2 sensor signal wire to get live readings. If the readings are below spec, you’ve found the most likely problem—either the heater isn’t generating enough heat or the sensor is worn/contaminated.
Or, swap the bank 1 sensor with the bank 2 sensor. If the code follows to the new location, it’s the sensor.
©, 2018 Rick Muscoplat
Posted on by Rick Muscoplat