Code
P1242
MINI
P — Powertrain
Manifold absolute pressure (MAP) sensor 2 - too high in deceleration
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EN: 0
RU: 1
AI status
Completed
Completed
100%
Causes
- Faulty MAP sensor 2 (internal failure or contamination)
- Open/short or poor connection in MAP sensor 2 wiring or connector
- Intake tract leak (hoses, intercooler, clamps, gasket) allowing pressure to remain high
- Blocked or malfunctioning bypass/recirculation or wastegate actuator causing retained boost
- Faulty boost control solenoid/valve (e.g., N75 or manufacturer equivalent) or its control circuit
- PCV or crankcase ventilation stuck open or hoses disconnected
Symptoms
- Illuminated MIL / Check Engine Light
- Reduced engine braking / higher-than-normal deceleration engine speed
- Hesitation or surging during transitions to closed throttle
- Irregular idle or stalling in some cases
- Possible reduced fuel economy or limp-home behavior if ECU restricts boost
What to check
- Read freeze frame and live data with a capable scan tool; note MAP2 value during closed-throttle decel and compare to MAP1 (if equipped)
- Visual inspection of MAP2 sensor, connector and wiring for damage, corrosion or looseness
- Inspect intake tract, intercooler pipes, clamps and vacuum hoses for leaks or loose connections
- Operate turbo wastegate / bypass valve and boost control solenoid and observe movement and response
- Smoke test intake and vacuum system to locate leaks
- Perform a backprobing test to confirm sensor supply voltage, ground and signal output with a multimeter or oscilloscope
Signal parameters
- Sensor supply: stable reference voltage (typically from ECU; verify presence at connector)
- Sensor ground: continuity to chassis/ECU ground required
- MAP2 signal: should drop (indicating vacuum) during deceleration — relative decrease compared to cruise; consult vehicle-specific data for absolute values
- Compare MAP1 vs MAP2: both should show similar trend during decel; large divergence indicates sensor or local plumbing issue
- Noisy, stuck or out-of-range MAP2 signal indicates sensor or wiring fault
Diagnostic algorithm
- Retrieve and record freeze-frame data and all stored codes. Verify P1242 is current and note conditions it set under (RPM, load, temp).
- With a scan tool, monitor MAP2 and MAP1 (if present) during a controlled deceleration. Confirm MAP2 remains higher than expected while throttle is closed.
- Visually inspect MAP2 connector and wiring for damage, corrosion, chafing or loose pins. Repair any wiring issues and clear codes to retest.
- Inspect intake and boost plumbing for leaks: clamps, intercooler, hoses, throttle body and PCV lines. Pressure-test or smoke-test the intake to find leaks.
- Manually actuate or command the boost control solenoid and wastegate/diverter valve while observing MAP2. If valves fail to actuate or MAP2 doesn't respond, probe the control circuit.
- Bench-test MAP2 (or apply a hand vacuum/pressure with a gauge) and observe signal output with scan tool or multimeter to verify correct response across range.
- If wiring and plumbing OK but MAP2 still reads high during decel while MAP1 behaves correctly, replace MAP2 sensor and retest.
- If replacement sensor does not clear fault, investigate turbo actuator, wastegate linkage, and boost control system (solenoid, hoses, ECU commands).
- After repairs, clear codes and perform road test under the same conditions that originally set the code to confirm repair.
Likely causes
- Damaged or contaminated MAP2 sensor (sticking diaphragm or internal short)
- Damaged vacuum/boost hose between manifold and sensor or leaking intake clamp
- Turbo wastegate stuck closed or actuator not moving (causing residual boost)
- Faulty boost control solenoid not dumping boost during decel
- Loose or corroded sensor connector or broken wire to sensor signal or ground
- Intake leak downstream of MAF or upstream of MAP producing inconsistent readings
Fault status
Status
MAP sensor 2 reports higher-than-expected intake pressure during deceleration (low vacuum). Possible sensor, wiring, intake leak, or boost-control fault.
Repair difficulty: Medium
Diagnostic time: 1-3 hours
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