What Does P209C Mean?
The reductant injection air pressure sensor B signal voltage is below the minimum threshold expected by the control module. This indicates either a true low pressure condition or an electrical fault causing low voltage.
Common Causes
30%
Faulty air pressure sensor B reading low
25%
Short to ground in sensor signal wire
25%
DEF air pump failure resulting in low or no air pressure
12%
Damaged or corroded sensor connector causing poor connection
8%
Major air supply leak in lines or fittings
Diagnostic Steps
1
Step 1: Measure sensor B signal voltage at PCM connector with key on, engine off (should be approximately 0.5-1.0V at rest)
2
Step 2: Check for short to ground on signal wire by disconnecting sensor and measuring circuit resistance
3
Step 3: Apply 5V supply and ground to sensor and verify proper voltage output
4
Step 4: Command DEF air pump on with scan tool and verify pressure builds at sensor
5
Step 5: Inspect air supply system for major leaks while pump is operating
Estimated Repair Cost
$200 - $1,000
Parts + labor, varies by vehicle and location
The reductant injection air pressure sensor B signal voltage is below the minimum threshold expected by the control module. This indicates either a true low pressure condition or an electrical fault causing low voltage.
The most common cause of P209C (Reductant Injection Air Pressure Sensor "B" Circuit Low) is: Faulty air pressure sensor B reading low
Typical repair costs for P209C range from $200 to $1,000, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Vehicle is safe to operate but DEF system will not function correctly. May trigger emissions warning lights and eventual power reduction.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P209C to identify the root cause.
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Quick Info
Category
Powertrain
System
Diesel Emissions/SCR System
Difficulty
Type
Generic (SAE)
Recommended Tools
OBD2 Scanner
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