Transmission Fluid Pressure Sensor/Switch A Circuit High
The P0843 code means the vehicle's onboard diagnostic system has detected: Transmission Fluid Pressure Sensor/Switch A Circuit High. This is a moderate severity code.
- Keep driving?
- Short distances only
- DIY difficulty
- hard
- Estimated cost
- $100 - $400 for an external sensor or wiring repair; $500 - $1,000+ when the fault is internal (e.g. a GM 6L80/6L90 TEHCM pressure-switch failure requiring a pan drop and relearn, or a CVT valve body on Nissan/Subaru applications)
Common Misconceptions / Fact Check
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Commonly claimedSome sources say P0843's harsh shifts come from the TCM reducing line pressure.
VerifiedIt is the opposite. P0843 is an electrical circuit fault (a high signal, not real pressure); the TCM falls back to a default strategy that commands MAXIMUM line pressure -- and that high pressure is what produces the firm or harsh shifts.
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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Common Symptoms
- Check Engine Light illuminated
- Harsh or firm shifts
- Transmission may not upshift properly
- Vehicle may enter limp mode
Probable Causes (Ranked by Likelihood)
- Open circuit in pressure sensor A ground wire Very High
- Faulty pressure sensor A reading high High
- Short to voltage in sensor signal wire Moderate
- TCM reference voltage issue Low
Step-by-Step Diagnostic Procedure
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Measure voltage at the pressure sensor A connector with key on, engine off
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Check for an open ground wire at the sensor
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Test for short to battery voltage on the signal wire
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Replace sensor A if wiring checks pass
Common Fixes by Vehicle
What techs usually find when diagnosing P0843 on specific vehicles — tap your vehicle for the fix and the exact part:
2007-2018 Chevrolet Silverado/GMC Sierra/Tahoe & Camaro/Cadillac CTS (6L80/6L90) Hard DIY
P0843 means the transmission fluid pressure (TFP) sensor/switch 'A' circuit is reading HIGH voltage -- typically an open circuit, a disconnected sensor, or low/aerated fluid, not always a bad sensor. On the 6L80/6L90 the pressure switches live inside the valve body / TEHCM, so check fluid level and condition and inspect the internal wiring harness and pass-through connector first. If the sensor itself is bad it requires a pan drop, and many cases need the control-solenoid (TEHCM) assembly plus a relearn.
Labor: 2-4 hours2009-2019 Ford F-150/Expedition/Mustang (6R80 6-speed) Hard DIY
On the 6R80, P0843 is a HIGH-voltage fault on the transmission fluid pressure sensor circuit. Because high voltage usually means an open or out-of-range signal, inspect the internal main control (valve body) wiring harness and the case connector before condemning the sensor -- a chafed lead or a loose connector is cheaper than a teardown. Verify fluid level and condition (Mercon LV) first. The sensor is internal, so a pan drop and fluid refill are part of the job.
Labor: 2-4 hoursMost automatic transmissions (general diagnosis) Hard DIY
P0843 = Transmission Fluid Pressure Sensor/Switch 'A' Circuit HIGH. Work cheapest-to-most-expensive: (1) check fluid level and condition -- low or burnt fluid skews pressure readings; (2) inspect the sensor connector and wiring for corrosion, chafing, or an open circuit, since 'circuit high' often means an open rather than a failed sensor; (3) only then replace the fluid pressure sensor/switch, which on most units is internal and requires dropping the pan. Clear codes and road-test to confirm before assuming an internal failure.
Labor: 1-4 hoursFrequently Asked Questions
What causes a high signal from the pressure sensor?
A high signal typically means the sensor ground is open or the signal wire is shorted to battery voltage, causing the TCM to see a falsely high pressure reading.
Why does high pressure cause harsh shifts?
P0843 is a circuit fault, so the high signal is electrical rather than real pressure. Once the code sets, the TCM can no longer trust the pressure reading and falls back to a default strategy, commonly commanding maximum line pressure. That high line pressure is what produces the firm or harsh shifts; the TCM relies on accurate pressure data for smooth shifting.
Is P0843 the opposite of P0842?
Yes, P0842 indicates a low signal and P0843 indicates a high signal from the same pressure sensor circuit. The causes are complementary.
Sources
This page is built from documented references. Verify against your own service info before repair work.
P0843 on Diesel, EV & Equipment
The same code ID appears across other engines and platforms. The diagnostic flow varies by manufacturer — these are the platform-specific breakdowns:
See All Codes & Fixes for Your Vehicle
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