P1128 on Honda: How to Diagnose & Fix
Closed Loop Fueling Not Achieved, Bank 1
1 Honda model · 1 documented issue · $40-$180 repair
P1128 on Honda by model: symptoms, diagnosis and fix
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.
Common Symptoms
- Check engine light with P0107, P0108, P1128 or P1129
- Rough idle and stumbling
- Hesitation and loss of power under load
- Rich-running condition and rotten-egg smell from catalyst
- Hard cold starts and emissions test failure
How to Fix
Verify the vacuum supply hose at the throttle body is not cracked or off, then back-probe the MAP sensor signal wire: at key-on engine-off it should read ~2.8-3.0V (atmospheric), dropping to ~1.0-1.5V at idle. If voltage is stuck high, stuck low, or doesn't track manifold vacuum, replace the MAP sensor (Honda P/N 37830-P05-A01 or equivalent). Clear codes and re-test; persistent codes after replacement usually indicate a vacuum leak.
What you need to fix it
Only diagnosis- and fitment-reviewed repair parts are linked here.
- Genuine Honda 37830-PAA-S00 MAP sensor setOEMHonda 37830-PAA-S00 (replaces 37830-P0G-S00)
MAP sensor replacement after reference, ground, vacuum passage and signal-response tests prove sensor failure Fitment/availability: Honda catalog fitment covers 1996-2000 Civic DX/EX/HX/Si/CX/GX/LX branches and supersedes 37830-P0G-S00. …
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Sources for this issue (3)
P1128 code reference
General diagnostic-code context. The code alone does not confirm a cause for your vehicle; use the applicable issue card and its sources.
Frequently asked questions
What does P1128 mean on a 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 it is documented across 1 model: Civic.
What are the most common causes of P1128?
The most common causes of P1128 are: 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. The specific cause varies by vehicle.
How much does it cost to fix P1128 on a Honda?
Published repair estimates on Honda range from $40 to $180 depending on the model and root cause. If an issue mentions a recall or campaign, check your VIN with the manufacturer or NHTSA and confirm eligibility and coverage with a dealer.
Looking for P1128 on a different make?
View P1128 across all makes →Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.