P1128 on Honda
Closed Loop Fueling Not Achieved, Bank 1
P1128 on Honda vehicles indicates closed loop fueling not achieved, bank 1. Au7o has documented this code across 1 Honda model — most commonly on Civic. A Chrysler/Dodge/Jeep/Ram manufacturer-specific code. The powertrain control module never saw the Bank 1 upstream oxygen sensor reach the conditions needed to switch from open loop (preprogrammed fuel mixture) into closed loop (mixture corrected from live sensor data) within the allowed time. Running in open loop means the engine is guessing at fuel delivery, so expect worse fuel economy, rough running, and a failed emissions test. Other makes assign P1128 to entirely different faults - Honda uses it for "MAP Sensor Signal Lower Than Expected" and Ford uses it for upstream O2 sensor connectors swapped bank to bank - so confirm against the make. Typical repair costs on Honda range from $40 to $180, depending on the specific model and root cause.
Common Causes of P1128
- •Failed or slow-responding upstream (Bank 1) oxygen sensor
- •Weak fuel delivery - clogged fuel pump strainer, weak pump, or restricted filter
- •Engine thermostat stuck open, so the engine never reaches operating temperature
- •Exhaust leak ahead of the upstream O2 sensor
- •Failed MAP, MAF, or intake air temperature sensor
- •Wiring or connector faults in the O2 sensor circuit
P1128 on Honda by Model
Honda Civic(1 issue)
- MAP Sensor Failure (1.6L D-Series)1996-1999
The manifold absolute pressure (MAP) sensor mounted on the throttle body of 1.6L D16 Civics commonly fails as it ages, losing the ability to hold vacuum or drifting out of its 0.5-4.5V calibration range. Because the PCM uses MAP voltage as the primary load input on these speed-density systems, a failing sensor causes the ECU to mis-fuel and mis-time the engine. Failures often set P0107 / P0108 / P1128 / P1129 codes and frequently coincide with cracked vacuum hoses on the upper intake.
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View P1128 across all makes →Frequently Asked Questions
What does P1128 mean on Honda?▼
P1128 stands for "Closed Loop Fueling Not Achieved, Bank 1." A Chrysler/Dodge/Jeep/Ram manufacturer-specific code. The powertrain control module never saw the Bank 1 upstream oxygen sensor reach the conditions needed to switch from open loop (preprogrammed fuel mixture) into closed loop (mixture corrected from live sensor data) within the allowed time. Running in open loop means the engine is guessing at fuel delivery, so expect worse fuel economy, rough running, and a failed emissions test. Other makes assign P1128 to entirely different faults - Honda uses it for "MAP Sensor Signal Lower Than Expected" and Ford uses it for upstream O2 sensor connectors swapped bank to bank - so confirm against the make. On Honda specifically, this code is documented across 1 model.
What causes P1128 on Honda vehicles?▼
Common causes on Honda: Failed or slow-responding upstream (Bank 1) oxygen sensor, Weak fuel delivery - clogged fuel pump strainer, weak pump, or restricted filter, Engine thermostat stuck open, so the engine never reaches operating temperature, Exhaust leak ahead of the upstream O2 sensor, Failed MAP, MAF, or intake air temperature sensor. Specific causes vary by model and year — see the per-model sections below.
How much does it cost to fix P1128 on a Honda?▼
Repair costs on Honda range from $40 to $180, depending on the specific model and root cause.
Which Honda models have P1128 documented?▼
Au7o has documented P1128 on 1 Honda model: Civic.