P0849 moderate

Transmission Fluid Pressure Sensor/Switch B Circuit Intermittent

The P0849 code means the vehicle's onboard diagnostic system has detected: Transmission Fluid Pressure Sensor/Switch B Circuit Intermittent. This is a moderate severity code.

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Keep driving?: Short distances only
DIY difficulty: hard
Estimated cost: $150 - $800 (parts and labor)

Check Your Fluid First

ANCEL AD310 OBD-II Scanner

Read and clear transmission codes. Many trans codes are caused by low or burnt fluid -- check level and color before assuming the worst.

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Read Transmission Codes

LAUNCH CRP123X (Reads All Systems)

Basic scanners only read engine codes. This reads transmission, ABS, and airbag too.

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Common Symptoms

Check Engine Light on
Intermittent harsh shifting
Transmission pressure reading drops out
Occasional shift quality issues

Probable Causes (Ranked by Likelihood)

Loose connector at transmission fluid pressure sensor B Very High
Intermittent wiring break in pressure sensor B circuit High
Pressure sensor B with marginal internal connection Moderate
Transmission fluid contamination affecting sensor Moderate

Step-by-Step Diagnostic Procedure

Wiggle the pressure sensor B connector while monitoring signal with scan tool
Inspect sensor wiring for intermittent breaks, especially at connectors
Monitor pressure sensor B data over extended driving to capture dropouts
Check transmission fluid condition; debris can affect sensor readings

Frequently Asked Questions

What causes intermittent transmission pressure sensor issues?

The most common cause is a loose or corroded connector. Transmission sensors operate in a high-vibration, high-temperature environment that degrades connections over time.

Can contaminated fluid cause intermittent sensor readings?

Yes. Metallic debris in old transmission fluid can create intermittent electrical paths or block pressure passages, causing erratic sensor readings.

Should I change the fluid when replacing a pressure sensor?

Yes. If the fluid is contaminated, the debris can damage the new sensor. A fluid change also removes contaminants that may have contributed to the original sensor failure.