What does it mean?
P1128 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Manifold Absolute Pressure (MAP) Sensor Signal Lower Than Expected .
Symptoms
- Malfunction indicator lamp on with noticeably reduced power under acceleration
- Hesitation, surging or a heavy, sluggish feel when pulling away
- Rough idle and occasional stalling
- Black smoke or a fuel smell if the mixture is thrown rich
- Hard starting, especially when the engine is warm
Likely causes
- Restricted air intake: heavily clogged air filter, collapsed intake duct or blocked snorkel
- Carbon or oil sludge plugging the MAP sensor pressure port or its vacuum passage
- Cracked, split or disconnected vacuum hose feeding a remotely mounted MAP sensor
- Throttle plate not opening fully because of a binding linkage or a coked bore
- Restricted exhaust from a collapsed catalytic converter or crushed pipe raising pumping losses
- Drifted sensor element giving a low biased signal across the whole range
- High resistance in the signal or ground circuit, dropping the voltage seen by the PCM (powertrain control module)
How to fix it
- Compare MAP live data with barometric pressure at key on, engine off; the two should read almost the same
- Log MAP at idle and under snap throttle and compare it with a mechanical vacuum gauge reading
- Remove and inspect the air filter and the full intake tract for restriction or a collapsed duct
- Pull the MAP sensor and clean carbon from its port; check the vacuum hose for cracks and blockage
- Measure the 5 V reference, the signal and the ground at the sensor connector with the key on
- Check exhaust back pressure if the vacuum reading is low and steady at higher engine speeds
- Replace the sensor only once the intake, exhaust and wiring have been proven correct, then recheck fuel trims