P0842
Transmission Fluid Pressure Sensor/Switch A Circuit LowP0842 is a generic OBD-II powertrain diagnostic trouble code: Transmission Fluid Pressure Sensor/Switch A Circuit Low. It is logged by the engine control unit when the trans monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.
What P0842 means
P0842 is an SAE generic powertrain code indicating that the signal voltage from Transmission Fluid Pressure (TFP) Sensor/Switch A has fallen below the minimum expected threshold — a 'circuit low' condition. The TCM uses line pressure feedback from this sensor to verify that the hydraulic circuit has built adequate pressure before commanding gear changes. When the reported voltage remains at or near zero (or below a calibrated low-voltage limit) for longer than the allowed rationality window, P0842 is set and the MIL illuminates.
The root cause can be an open or short-to-ground in the sensor signal wire, a failed sensor whose internal element has shorted, a damaged or corroded connector at the sensor or TCM, or a loss of the 5 V reference supply that the sensor requires. It is also possible — though less common — for actual low line pressure (caused by a worn pump, solenoid, or low fluid level) to drive the sensor output into the low range; in that case the sensor is accurately reporting a real hydraulic problem.
Because the TCM relies on pressure feedback to determine correct clutch engagement timing, an incorrect low-pressure reading can cause the TCM to delay or prevent shifts, trigger limp mode, or misdiagnose shift solenoid performance.
Common causes
Most-frequently reported root causes when P0842 is logged.
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1
Failed TFP Sensor/Switch A with internally shorted element producing near-zero voltage output
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2
Short-to-ground on the sensor signal wire between the sensor and TCM
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3
Open circuit or broken wire in the 5 V reference supply to the sensor
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4
Corroded, damaged, or unseated connector at the sensor or the TCM pin
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5
Low transmission fluid level leading to genuine low hydraulic line pressure
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6
Worn transmission pump unable to maintain minimum line pressure
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7
Faulty TCM input circuit for the pressure sensor channel
Symptoms drivers notice
How to diagnose P0842
A typical diagnostic flow when this code is present.
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1
Record all DTCs and freeze-frame data; note companion transmission or pressure-related codes
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2
Check transmission fluid level and condition — correct any low or contaminated fluid before proceeding
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3
Inspect the TFP sensor connector and harness for corrosion, bent pins, or chafing damage; repair as needed
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4
With ignition on (engine off), backprobe the sensor signal wire and confirm the 5 V reference is present; a missing reference points to a TCM or wiring fault upstream
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5
With the engine running and transmission in gear, observe live sensor voltage/pressure data via a scan tool — confirm whether the reading is always at the low-voltage floor or fluctuates with actual pressure demands
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6
If the electrical circuit tests normal, connect a mechanical line-pressure gauge to the service port and compare actual hydraulic pressure to specification across operating ranges
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7
Replace the TFP sensor if electrical circuit is intact but sensor output remains implausibly low; address any hydraulic root cause (pump, fluid, solenoid) if mechanical pressure is confirmed deficient
Related powertrain codes
- P0218 — Transmission Over Temperature Condition
- P0700 — Transmission Control System Malfunction
- P0701 — Transmission Control System Range/Performance
- P0702 — Transmission Control System Electrical
- P0703 — Torque Converter/Brake Switch B Circuit Malfunction
- P0705 — Transmission Range Sensor A Circuit malfunction (PRNDL Input)
Frequently asked questions
Does P0842 always mean the pressure sensor itself is bad?
No. The code only means the circuit voltage is low. A short-to-ground anywhere in the signal wire, a failed 5 V reference, or a corroded connector can all produce the same low-voltage reading without the sensor itself being defective. Test the circuit first.
Can low transmission fluid trigger P0842?
Yes. If actual hydraulic line pressure drops sufficiently low — due to low fluid level, a worn pump, or internal leakage — the pressure sensor accurately reads low and sets P0842. Always verify fluid level and condition before assuming an electrical fault.
Is it safe to drive with P0842 active?
Limited driving in an emergency is possible, but the transmission may be in limp mode. If the code reflects actual low line pressure rather than a sensor fault, continued driving risks clutch damage. Have the vehicle inspected promptly.
Where is TFP Sensor A typically located?
On most vehicles it is threaded into the transmission case or valve body and accessible from underneath the vehicle without dropping the transmission pan. Location varies by make and model.
Disabling P0842 in software
RaceTune can permanently disable P0842 — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.
Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.
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