P1128: How to Diagnose & Fix — Closed Loop Fueling Not Achieved, Bank 1
4 vehicles · 2 makes · $40-$180 repair
How to diagnose and fix P1128: most-reported vehicles
Ranked by owner reports. Each card gives the symptoms, how to confirm the cause, the fix, parts and sources for that vehicle.
Honda Civic · 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.
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)
Audi SQ5 · 2014–2025
Audi states that contaminated gasoline or gasoline with insufficient deposit-control additives can contribute to deposits in injectors, combustion chambers, the intake manifold and intake valves, with rough running, hesitation, reduced performance, poor fuel economy or related ECM faults. This does not establish a universal SQ5 intake-valve defect, a mandatory walnut-blasting interval or a fixed mileage. Audi separately warns that unapproved fuel additives are not permitted on 2018-2025 EA839 V6 TFSI SQ5 models because a negative effect on engine life cannot be ruled out.
Common Symptoms
- Rough running after a cold start
- Excessive engine cranking time
- Hesitation while driving
- Rough idle
- Reduced engine performance
- Poor fuel economy
- MIL with misfire, lean-system or air/fuel-ratio imbalance faults
How to Fix
Confirm fuel quality, symptoms and stored DTCs before replacing or cleaning components. If contaminated gasoline is suspected, advise changing the fuel source; if low detergent quality is suspected, Audi recommends Top Tier detergent gasoline. Continue workshop diagnosis if symptoms persist. On 2018-2025 EA839 SQ5 models, do not recommend unapproved cleaners or additives; follow the owner manual and use only Audi-approved petrol additives. Do not prescribe routine walnut blasting, a catch can or chemical cleaning from this record.
Sources for this issue (2)
Audi A4 · 2009–2021
Audi states that contaminated gasoline or gasoline with insufficient deposit-control additives can contribute to deposits on intake valves, the intake manifold, fuel injectors and combustion chambers. Possible results include rough running after cold start, extended cranking, hesitation, rough idle, reduced performance, poor fuel economy or an MIL with supported misfire, lean-system or cylinder-imbalance faults. This does not establish a universal A4 2.0T intake-valve defect, mandatory walnut-blasting interval or fixed failure mileage.
Common Symptoms
- Rough running after a cold start
- Excessive engine cranking time
- Hesitation while driving
- Rough idle
- Reduced engine performance
- Poor fuel economy
- MIL with supported misfire, lean-system or cylinder-imbalance faults
How to Fix
Confirm that the vehicle is gasoline-powered, document fuel quality, symptoms and stored DTCs, and follow current guided diagnosis before cleaning or replacing anything. If contaminated gasoline is suspected, change fuel source; if detergent quality is suspect, Audi recommends Top Tier detergent gasoline. Identify the actual deposit location before selecting a cleaning process. Do not prescribe routine walnut blasting, a catch can, chemical cleaner or high-RPM driving from this record.
Sources for this issue (1)
Audi Q5 · 2009–2025
Find P1128 by make
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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?
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.
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?
Repair costs for P1128 range from $40 to $180, depending on the vehicle and root cause.
Which vehicles are affected by P1128?
Au7o has documented P1128 across 4 vehicle models from 2 manufacturers: Audi, Honda.
Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.