Ford P1112: MAP Sensor or Dual Alternator Circuit
The P1112 code is a Ford-specific diagnostic trouble code that indicates a fault in the lower circuit of the dual alternator system or an intermittent low voltage condition in the Manifold Absolute Pressure (MAP) sensor circuit. The PCM sets this code when the MAP sensor signal intermittently drops below the minimum expected threshold, or when there is an anomaly in the secondary alternator control circuit on dual alternator applications. The MAP sensor measures intake manifold pressure, which is critical for calculating engine load, fuel injection timing, and ignition advance.
01. Most Common Causes
- Faulty or worn MAP sensor
- Corroded or loose electrical connections in the MAP sensor circuit
- Intermittent short to ground in the MAP sensor wiring
- Failed secondary alternator (on dual alternator systems)
- Defective secondary alternator relay
- Internal PCM failure (less common)
- Intermittent vacuum leaks in the intake manifold or connected hoses
02. Symptoms You Will Notice
- Check Engine Light illuminated
- Poor engine performance or hesitation
- Reduced fuel economy
- Rough idle or stalling
03. Step-by-Step Diagnosis
Scan for codes and record freeze frame data
Inspect MAP sensor connector and wiring for corrosion, damage, or loose connections
Test MAP sensor signal voltage with a scan tool or multimeter while monitoring for intermittent drops
Check for vacuum leaks using a smoke machine or propane enrichment method
On dual alternator systems, test secondary alternator output and relay operation
If all else fails, perform a PCM self-test or consider PCM replacement
1. Frequently Asked Questions
Is it safe to drive?▼
You can usually drive short distances, but the vehicle may experience reduced performance and fuel economy. It's best to diagnose and repair the issue promptly to avoid potential stalling or further damage.
How much does it cost to fix?▼
Repair costs vary widely depending on the root cause. A MAP sensor replacement typically ranges from $100 to $200 including parts and labor. If the secondary alternator or relay is at fault, costs can range from $200 to $600 or more.