P0842 is a more specific pressure code than P0841. Where P0841 indicates the pressure sensor is reading out of its expected range, P0842 specifically means the sensor A circuit is reading below the expected minimum threshold - the TCM is seeing a signal that indicates pressure is too low, or the circuit itself has dropped below minimum voltage.
P0841 vs P0842 vs P0843
- P0841: Pressure sensor A range/performance - signal is erratic or doesn't match conditions
- P0842: Pressure sensor A circuit low - signal is below minimum threshold (often a circuit fault or very low actual pressure)
- P0843: Pressure sensor A circuit high - signal is above maximum threshold
P0842 is the most concerning of the three because it often indicates either a short-to-ground wiring fault or actual low hydraulic pressure - both of which need immediate attention.
Common Causes of P0842
- Short to ground in the sensor circuit: A damaged wire that's touching the chassis or transmission case pulls the signal circuit below the minimum voltage threshold, causing the TCM to read the pressure as zero or near-zero.
- Failed pressure sensor/switch (shorted internally): The sensor itself develops an internal short, producing a continuously low voltage output.
- Actual low hydraulic pressure: If line pressure is genuinely very low (worn pump, severely clogged filter, very low fluid level), the sensor accurately reports low pressure and P0842 is set. This is the most serious scenario.
- Corroded sensor connector: Road salt corrosion on the connector increases circuit resistance, which can pull voltage down enough to set P0842.
Symptoms of P0842
- Check engine light (P0842 stored, often with P0700)
- Slipping shifts or delayed gear engagement if actual pressure is low
- Transmission overheating (low line pressure causes clutch slip and heat generation)
- Limp mode - TCM may lock into a single gear to prevent damage from low-pressure operation
- No noticeable driving symptoms if the cause is purely a wiring fault (sensor reads low but actual pressure is normal)
Diagnosis
P0842 requires a two-step approach:
- Electrical testing first: Check circuit voltage at the sensor connector with the key on. Normal reference voltage is typically 5V. If voltage is below 1V before the engine starts, you have a circuit fault, not a pressure fault.
- Mechanical pressure test second: If circuit checks out, connect a calibrated hydraulic pressure gauge to verify actual line pressure against spec. This determines whether the sensor is accurately reporting a real problem or misreading normal pressure.
Chicago Repair Costs for P0842
- Wiring/connector repair: quoted after diagnosis, access check, and parts availability.
- Pressure sensor/switch replacement: quoted after diagnosis, access check, and parts availability.
- Transmission fluid service (if low/contaminated): quoted after diagnosis, access check, and parts availability.
- Transmission pump replacement (if actual low pressure is confirmed): quoted after diagnosis, access check, and parts availability.
- Full rebuild (if pump failure has caused clutch damage): quoted after diagnosis, access check, and parts availability.
Free diagnostic at Chicago Transmission - 2450 N Lincoln Ave. Call (312) 452-5637, Monday - Friday 7:30am - 6pm, Saturday 8am - 2pm.
