P0842 and P0843: transmission fluid pressure sensor A circuit

Modern automatic gearboxes run closed-loop pressure control: the module commands a pressure, a sensor reports what was achieved, and the command is corrected. That requires a trustworthy sensor.

Code	Meaning
P0840	Transmission fluid pressure sensor A circuit
P0841	Range/performance
P0842	Circuit low input
P0843	Circuit high input

Related: P0745 to P0747 pressure control solenoid A.

Fix the instrument before the machine

The practical rule for this whole family of codes.

If P0842 appears alongside slipping, gear ratio codes or pressure control codes, the sensor question comes first. A sensor reporting low pressure makes the module raise line pressure unnecessarily — or conclude the gearbox is failing when it is not.

Codes present	Order of work
P0842 alone	Sensor or circuit
P0868 alone	Investigate genuine low pressure
P0842 + slipping codes	Sensor first, then re-assess
P0842 + several other sensors	Shared reference or internal harness

The key-off test

Simple and decisive.

With the engine off and the gearbox unpressurised, transmission fluid pressure must read zero or near zero. There is no line pressure without a running engine driving the pump.

Key-off reading	Verdict
Zero	Sensor plausible
A significant pressure	Sensor faulty
Negative or pinned high	Circuit fault
Same value running and stopped	Stuck sensor

A sensor reporting pressure with the engine off has been caught being wrong, and nothing it says while running can be trusted.

Causes

The internal harness

The recurring theme of transmission electrical faults. Wiring inside the gearbox passes through a sealed case connector; the seal hardens with heat and age; fluid wicks along the wires.

Fluid inside the external transmission connector is a definite finding and explains several simultaneous codes without several failed parts.

The sensor

Often integrated into the valve body or the internal harness assembly, so not separately replaceable on many gearboxes. Establish this early, because it sets the cost of the repair.

The external connector

Corrosion, and heat from a nearby exhaust.

The shared reference

Sensors share a 5 V supply. One shorted sensor pulls the reference down and sets codes across several. A crowd of sensor codes is a reference problem, not a crowd of failures.

Genuine low pressure, if the sensor is good

Then the investigation moves on, in this order:

1. Fluid level. Low fluid means the pump draws air. On dipstick-less gearboxes the level is checked at a specified fluid temperature: P0710 to P0713 transmission fluid temperature sensor.
2. Fluid condition. Degraded fluid, a blocked filter, or debris in the pan.
3. The pressure control solenoid, which sets the pressure.
4. The valve body, where bore wear leaks pressure internally.
5. The pump, and finally internal seal wear.

Automatic transmission ATF diagnostics and P0868 transmission fluid pressure low.

Symptoms

• Harsh or soft shifts, depending on which way the reading is wrong.
• Slipping under load if pressure is genuinely low.
• Limp mode, holding a single gear: limp mode, what it is and how to exit.
• Delayed engagement when selecting Drive.
• Occasionally nothing noticeable, with the code appearing on a routine scan.

What generic OBD-II reaches

The codes, and often commanded and actual gear. Line pressure values, solenoid duty and adaptation data usually need enhanced access to the transmission module.

P0700 transmission control system malfunction and scan tool limits.

The key-off pressure reading, though, is available wherever the value is exposed at all — and it is the single most useful check in this article.

In what order

1. Read pressure with the engine off.
2. Read all codes — a crowd means a shared cause.
3. Inspect the external connector for fluid and corrosion.
4. Check fluid level and condition.
5. Establish whether the sensor is separately replaceable.
6. Only then investigate pressure generation itself.

You can check this yourself. Pair an ELM327 adapter, open the free tier of the app and read transmission line pressure with the ignition on and the engine off. If that number is not zero, every pressure-related conclusion about your gearbox is suspect.