Originally posted by snooprobbyrob:
they're both upstream codes,



Yes, we understand that. The question is whether the lowers were replaced instead of the uppers. But let's go back to the TSB.

HEATED OXYGEN SENSOR (HO2S) MONITOR - HO2S SIGNAL CIRCUIT MALFUNCTION
Diagnostic Trouble Code Description Possible Causes
P0131 - HO2S Sensor Circuit Out of Range Low Voltage (HO2S-11) The HO2S sensor is monitored for a negative voltage known as Characteristic Shift Downward (CSD). If the sensor is switching from 0 volts to -1 volts during testing and DTC P0131 is present, the PCM will be in FMEM.

Electrical:
�· Contaminated HO2S sensor or connector (coolant, water, silicone, fuel, oil, etc.)
�· Chafed/damaged wiring
�· Crossed HO2S signal/signal return wiring
�· PCM

HEATED OXYGEN SENSOR (HO2S) MONITOR - HO2S SIGNAL CIRCUIT MALFUNCTION
Diagnostic Trouble Code Description Possible Causes
P0155 - HO2S Sensor Heater Circuit Malfunction (HO2S-21) (Bank 2 Upper)

During testing, the HO2S heaters are checked for opens/shorts and excessive current draw. The code will set when current draw exceeds a maximum calibrated limit or falls below a minimum calibrated limit and/or an open or short is detected. �· Blown fuse
�· Short to VPWR in harness or HO2S
�· Water in harness connector
�· Open VPWR or GND circuit
�· Low battery voltage
�· Poor electrical connections from PCM to HO2S sensor
�· HO2S heater
�· PCM

So based on what Ray and you are saying, it could be that
it was due to running rich (even though you did not get a P0172 and P0175 code).

"Always do the cheap and easy ones first." 1996 V6 ATX 96K miles