Vacuum leak from a cracked or disconnected intake hose, intake manifold gasket, or vacuum line; Contaminated or failing mass airflow sensor sending a lean-biased signal; Weak or failing fuel pump delivering insufficient volume; Clogged or restricted fuel filter; Leaking or stuck-open fuel injector on bank 1; Faulty upstream oxygen sensor on bank 1 giving an inaccurate reading; Exhaust leak before the upstream oxygen sensor allowing outside air into the exhaust stream

DTC codes / Mazda codes / P1133

P1133 · MazdaBank 1 Fuel Control Shifted Lean

Q: What does code P1133 mean?

Bank 1 Fuel Control Shifted Lean. P1133 on a Mazda indicates that the powertrain control module has detected the bank 1 fuel control system has shifted lean beyond its calibrated limit, meaning the engine is receiving too much air or not enough fuel on the bank containing cylinder 1. The computer monitors the long-term fuel trim and sets this code when the correction needed to maintain the proper air-fuel ratio exceeds a programmed threshold for a set time.

P1133 on a Mazda indicates that the powertrain control module has detected the bank 1 fuel control system has shifted lean beyond its calibrated limit, meaning the engine is receiving too much air or not enough fuel on the bank containing cylinder 1. The computer monitors the long-term fuel trim and sets this code when the correction needed to maintain the proper air-fuel ratio exceeds a programmed threshold for a set time.

SystemPowertrain

Code typeMazda specific

SeverityMedium

Fix difficultyIntermediate

AreaFuel and air metering

Symptoms you may notice

Illuminated check engine light, often with no other noticeable symptoms
Rough idle or intermittent misfire, especially when the engine is cold
Hesitation or stumble during acceleration
Reduced fuel economy compared to normal
Occasional surging or uneven power delivery
Failed emissions test due to high nitrogen oxide or carbon monoxide levels

Common causes, most likely first

Vacuum leak from a cracked or disconnected intake hose, intake manifold gasket, or vacuum line
Contaminated or failing mass airflow sensor sending a lean-biased signal
Weak or failing fuel pump delivering insufficient volume
Clogged or restricted fuel filter
Leaking or stuck-open fuel injector on bank 1
Faulty upstream oxygen sensor on bank 1 giving an inaccurate reading
Exhaust leak before the upstream oxygen sensor allowing outside air into the exhaust stream

How to diagnose and fix it

Connect a scan tool and record freeze-frame data and long-term fuel trim values to confirm the lean condition
Perform a visual inspection of all intake and vacuum lines for cracks, loose clamps, or disconnected hoses
Use a smoke machine to pressurize the intake system and locate any hidden vacuum leaks
Check fuel pressure with a gauge and compare to Mazda specifications; replace the fuel filter if pressure is low
Clean the mass airflow sensor with a dedicated cleaner and verify its live data readings against expected values
Inspect the exhaust system for leaks upstream of the bank 1 oxygen sensor and repair as needed
If all above checks pass, test the upstream oxygen sensor response and replace if sluggish or biased

Hidden causes of a lean bank 1 code

A dirty mass airflow sensor often reports less air than actually enters the engine, causing the computer to add fuel and eventually set P1133 without any driveability complaint.
Exhaust leaks between the cylinder head and the upstream oxygen sensor can pull in outside air during low-pressure pulses, fooling the sensor into reporting a lean condition that does not exist in the combustion chamber.
A weak fuel pump may only show up under load; a static fuel pressure test can pass while volume and pressure drop during acceleration, so a dynamic flow test is more reliable.
On some Mazda engines, a leaking intake manifold gasket at the number one cylinder runner creates a lean condition that is worse when cold and may not trigger a misfire code.
P1133 can be set by a lazy upstream oxygen sensor that responds slowly to fuel changes, even though it still switches rich and lean; comparing its switching frequency to the downstream sensor helps confirm.

Can I keep driving with P1133?

You can usually drive the vehicle short distances at moderate speeds, but avoid hard acceleration, heavy loads, and sustained high-speed driving until the lean condition is repaired. Prolonged driving with a lean mixture can overheat the catalytic converter and cause expensive damage.

Tools that help

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Guidance is general and written for Mazda vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.