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P0842 is a generic OBD-II powertrain code: Transmission Fluid Pressure Sensor/Switch A Circuit Low. It sets when the transmission control module monitors a hydraulic pressure switch (switch "A") and sees the circuit reading low voltage, or reading "pressure present," at a moment when the calibration says it should be high or open.
On most GM, Ford, and Chrysler automatics, these pressure switches live inside the valve body and toggle open/closed as line pressure rises and falls in a given circuit. The TCM cross-checks the switch state against the gear it commanded. When the switch state and the commanded state disagree in the "low" direction, you get P0842. Its sibling code P0843 is the same switch reading high.
This is not always a bad sensor. In fact, the single most common real-world cause is low or degraded transmission fluid dropping line pressure below the switch's trip point. Start there before you touch a wire.
What P0842 Actually Means, In Plain Terms
Inside the valve body, a fluid pressure switch is a normally-open or normally-closed contact that grounds a signal wire when hydraulic pressure pushes on a diaphragm. The TCM feeds the switch a 5V or 12V reference and watches whether the return line pulls to ground.
- Circuit "Low" means the TCM sees the return pulled low (near 0V) when its model of the transmission says it should be sitting high.
- That happens either because the switch genuinely closed early (real hydraulic problem) or because the wiring is shorted to ground, or because the connector is corroded and leaking to the case.
The code alone does not tell you which. That is the whole diagnostic job: separate a hydraulic cause from an electrical cause.
Root Causes, Ranked By How Often We See Them
| Cause | How Common | Tell-Tale Sign |
|---|---|---|
| Low / burnt / old fluid | Very common | Level low on dipstick, dark or burnt smell, shifts also feel soft |
| Shorted or chafed harness / internal wiring | Common | Code sets cold, intermittent, wiggle-test reproduces it |
| Corroded case connector / pass-through | Common | Green crust on pins, seepage at the connector, sets after fluid change |
| Failed pressure switch itself | Occasional | Switch resistance out of spec, all wiring good |
| Valve body / low line pressure | Occasional | Pressure gauge reads below spec, multiple pressure codes together |
| Failing TCM / internal driver | Rare | Everything else checks good, code follows the module |
Notice the top two are cheap. Do not let a shop sell you a valve body or a TCM before they have checked fluid level, fluid condition, and the wiring. That is the order a competent diagnostician works in.
Symptoms You Might Notice
- Check Engine Light on, sometimes with a flashing overdrive or "wrench" light.
- Transmission dropping into limp mode (stuck in one gear, usually 2nd or 3rd, no overdrive).
- Harsh or delayed shifts because the TCM defaults to max line pressure in limp mode.
- Speedometer and shifting may be unaffected on a purely electrical fault, since the trans still works mechanically.
A lot of drivers with P0842 report the car "drives fine but won't shift right on the highway." That points at limp mode, which is the TCM protecting itself once it stops trusting the pressure switch.
How To Diagnose It, Step By Step
1. Check fluid level and condition first
Bring the transmission to operating temperature and check the level exactly the way the factory procedure calls for. Many modern units have no dipstick and require a fill-plug check at a specified fluid temperature. Low fluid drops line pressure and trips the switch. Burnt, dark fluid means the hydraulics are already compromised. This ten-minute check solves a meaningful share of P0842s.
2. Scan for companion codes and freeze-frame
P0842 rarely lives alone. If you also see P0843, P0847/P0848 (switch B), or specific shift-solenoid codes, the pattern narrows the fault. Read the freeze-frame: what gear, what temperature, what RPM was commanded when it set? Cold-only faults lean electrical; hot-only faults lean hydraulic.
3. Test the wiring before condemning the switch
With the connector unplugged, back-probe and check: continuity from the TCM pin to the switch pin, resistance to ground (should be high/open when it should be open), and voltage on the reference wire key-on. A short to ground on the signal wire is the classic "circuit low." A quality scan tool that reads live transmission data lets you watch the switch state flip as you drive, which is the fastest way to catch an intermittent.
4. Put a mechanical gauge on line pressure
If wiring is clean and fluid is good, tap the line-pressure test port and confirm actual hydraulic pressure against the factory spec at idle and under load. Low pressure sends you to the pump, valve body, or a worn circuit, not the switch.
5. Test or replace the switch last
Only after wiring and hydraulics check out does the pressure switch itself become the suspect. Many are integrated into the solenoid/pressure-switch assembly, so on a lot of platforms you replace the whole pack rather than a single switch.
Need the Switch or Solenoid Pack? Ships Same-Day
Once you have confirmed the pressure switch or the solenoid assembly it lives in, gearboxinsider.com stocks the pressure switches, solenoid packs, and internal harnesses for the common GM, Ford, and Chrysler units. Same-day ship before 2pm Eastern, free shipping.
Shop Gearbox Insider →What The Repair Costs In 2026
| Fix | Parts | Total With Labor |
|---|---|---|
| Fluid and filter service | $40 - $120 | $150 - $350 |
| Repair chafed wiring / connector | $10 - $60 | $120 - $300 |
| Pressure switch (standalone) | $25 - $90 | $200 - $450 |
| Solenoid / pressure-switch pack | $120 - $400 | $450 - $900 |
| Valve body | $300 - $750 | $900 - $1,600 |
| TCM replace + reflash | $250 - $650 | $500 - $1,100 |
The spread is enormous, which is exactly why diagnosis order matters. Confirm the cheap causes are ruled out before anyone opens the transmission or orders a control module.
Recommended Tools For This Diagnosis
A bidirectional scan tool that reads live transmission data is the difference between guessing and knowing on P0842. You want to watch the pressure-switch state and line-pressure PID change in real time.
Check bidirectional scan tools
A back-probe pin set and a decent multimeter let you test the signal and reference wires without cutting insulation.
Check back-probe and meter kits
FAQ
Can I keep driving with P0842?
Short distances, at low speed, to get it diagnosed, usually yes. But in limp mode the transmission runs at max line pressure and lives in a single gear, which is hard on it and unsafe on the highway. Do not tow or road-trip on it.
Will a fluid change fix P0842?
Sometimes, if low or degraded fluid was the actual cause. It is the cheapest thing to rule out, so do it first, then clear the code and see if it returns.
Is P0842 the same as P0843?
Same switch, opposite direction. P0842 is the circuit reading low; P0843 is it reading high. If you see both, suspect wiring or the switch itself rather than hydraulics.