P1128 is a manufacturer-specific troubled code. In the case of the Chrysler Neon, it indicates that the engine did not enter “closed-loop” fueling on the bank 1 side.
If any other codes appear with P1128, start with them since this code does NOT indicate what has caused the engine not to go into closed-loop operation.
The most common cause of P1128 is a bad sensor (usually a bad oxygen sensor).
P1128 Definition: Closed Loop Fueling – Not Achieved – Bank 1
P1128 has the same meaning for all Chrysler-made vehicles. Here’s the three-part definition of P1128 in the Chrysler Neon:
Closed Loop Fueling
When you start your Neon, there are default factory air/fuel mixture settings that run off tables. This is open-loop fueling.
As data is received from the various engine monitoring sensors (O2, MAP, ECT, etc…), the Powertrain Control Module (PCM) uses it to optimize the air/fuel ratio in real-time. This is referred to as closed-loop fueling.
Not Achieved
The engine remains in closed-loop operation, usually due to a failure in one of the sensors mentioned above.
Bank 1
Bank 1 is the side of your Chrysler Neon’s engine with the first cylinder. You only need to worry about this if the engine is a “V” type with two cylinder heads. If there’s only one head, you don’t need to concern yourself with locating Bank 1.
On Chrysler engines with two banks, the side with the first cylinder can change depending on your engine. As a rule of thumb, bank one is on the passenger side of the engine on rear-wheel drive cars and trucks. On front-wheel drive vehicles, it’ll be on the opposite side when looking at the engine accessories. You can use this guide from aa1car to verify your engine.
Chrysler Neon P1128 Symptoms
Since P1128 tells you that your Neon vehicle has not entered closed loop operation, but not WHY it hasn’t, the symptoms can vary. Here are the common ones:
- Check engine light
- Decreased fuel mileage
- Stuttering or misfire
- Hard to start
- Fuel smell
Chrysler Neon P1128: Diagnosis
Check For Other Codes
The first order of business when diagnosing P1128 in the Neon is to check for other codes. It is doubtful you’ll get this code without another one being present.
P1128 is telling you the PCM is staying in open-loop mode. There should be another code that states what that issue is (such as P0136).
Oxygen Sensors
A defective upstream O2 sensor is probably the most common cause of P1128.
Oxygen sensors measure the exhaust gasses and are vital to the PCM’s ability to enter closed-loop operation.
A bad oxygen sensor on the bank 1 side is one of the most common causes of P1128. If you have an engine with two banks and an oxygen sensor-related trouble code, you can swap the bank one and bank two codes to see if it “jumps” to the other bank.
O2 Sensor Wiring
Often, a wiring issue can cause an O2 sensor to stop reporting correctly. Cracked, shorted, burnt, or otherwise, damaged wiring can cause it to stop talking to the PCM
MAP/MAF Sensor
The Manifold Absolute Pressure (MAP) sensor measures the air entering your Neon’s engine. The PCM can’t enter closed-loop if it’s not correctly calculating the airflow.
Engine Coolant Sensor
A bad ECT sensor can cause P1128 in MOPARS. The PCM adjusts the air/fuel mixture as the vehicle warms up. It can’t enter closed loop if it doesn’t know the vehicle is warm.
Other Causes of P1128
- If there are no other codes, suspect a vacuum or exhaust leak.
- Bad Idle Air Temp sensor
- Fuel pressure (too high or too low)
- Major engine mechanical failure
- Bad PCM (very rare)
Conclusion
A bad oxygen sensor is the most likely cause of P1128 in the Chrysler Neon. But, there are many other causes as well. If there are other codes, they can help you narrow it down.