This is one of the most frequent OBD2 trouble codes. Read the full article below to know what it means, how to fix it, and what other codes may show related to it.
P1129 is an OBD-II Code that refers to Downstream Oxygen Sensors Swapped from Bank to Bank (Ford, Lincoln, Mercury)
A P1129 code is a Ford, Lincoln, Mercury makes manufacturer specific diagnostic trouble code that occurs when the PCM/ECM receives a signal indicating that the oxygen sensor on the downstream side of the catalytic converter has been swapped from bank to bank.
DTC will set a P1129 code and illuminate the Check Engine Light when the PCM senses that the oxygen sensors have been swapped from bank to bank.
Common causes for this code include:
- When a mechanic installs an oxygen sensor in the wrong bank
- Wiring harness has been damaged
- Open or short between two oxygen sensors
- PCM/ECM has malfunctioned (rare)
The symptoms of a P1129 code are:
- Illuminated Check Engine Light
- Reduced engine performance
- Poor fuel economy
- Hesitation while accelerating
- Rough Engine idle
- Trouble codes related to the catalytic converter
To diagnose a P1129 DTC code, a technician would:
- Scan for codes in the ECM and look at the freeze frame data for failure.
- Visually inspect the wiring harness for damage.
- Check the oxygen sensor voltage signals on each bank and compare them to see if they are they are swapped.
- Test the PCM/ECM for proper function.
The following are some of the most common mistakes that a technician may make when diagnosing a P1129 code:
- Not thoroughly inspecting all the wiring and connectors for damage.
- Not checking the oxygen sensor voltage signals on each bank to see if they are in the wrong place.
- Testing the PCM/ECM for proper function when it may not be the cause of the code.
How serious is this?
A P1129 code is a fairly serious code as it can indicate a problem with the oxygen sensors or the wiring harness.
What repairs can fix the code?
The following are solutions that may fix this problem:
- Repairing or replacing any damaged wiring or connectors.
- Checking the oxygen sensor voltage signals on each bank and making sure they are not swapped.
- Testing the PCM/ECM for proper function.
- Replacing/swapping the oxygen sensors.
- Reprogramming the PCM/ECM.
A P1129 is related to and may be accompanied by the following codes:
P0030 – oxygen sensor circuit slow response (Bank 1 Sensor 1)
P0036 – oxygen sensor heater circuit malfunction (Bank 1 Sensor 2)
P0137 – oxygen sensor Heater Circuit Malfunction (Bank 2 Sensor 1)
P0138 – oxygen sensor Heater Circuit Malfunction (Bank 2 Sensor 2)
In conclusion, the P1129 code is a diagnostic trouble code that refers to a problem with the oxygen sensors or the wiring harness. The code will set a P1129 code and illuminate the Check Engine Light when the PCM senses that the oxygen sensors have been swapped from bank to bank. There are several possible repairs that may fix this problem, but the most common solution is to check the oxygen sensor voltage signals on each bank and make sure they are in the correct places and not swapped.