6T75 P0842: Fluid Pressure Switch 1 Circuit Low Causes & Fix

P0842 shows up most often on the GM Lambda crossovers: the Acadia, Outlook, Enclave and Traverse with the 3.6L and the 6T75. A generic scan tool reads it as "Transmission Fluid Pressure Sensor/Switch A Circuit Low." On this unit it means something narrower. GM's descriptor is Transmission Fluid Pressure Switch 1 Circuit Low Voltage, and per GM service information that switch reports on one valve only, the 3-5-Reverse clutch regulator valve.

That gives you a short list. On a Lambda, the 3-5-R circuit is also the one tied to GM's best-known 6T70/6T75 failure, the broken 3-5-R waved plate. So before you order a $600 TEHCM, work out whether the switch is lying or whether it is telling you the truth about a clutch circuit that is failing.

This post covers the 6T75 side of the code: which Lambda model years can set it at all, what GM's monitor is actually checking, the bench test that separates a bad module from a hydraulic fault, and the repair for each cause. The 6T70 and 6T75 share the same control solenoid valve assembly design and the same GM diagnostic, so where the procedure is identical I say so. The 6T70 version of this code covers the sedans and the Equinox.

Recommended 6T75 parts

The Gen. 1 control solenoid valve assembly that carries switch 1, the 2008-2012 valve body with the 3-5-R regulator, and pressure switch seal tooling. Secure checkout.

6T75 Solenoid Kit 2008

$587.99View part

6T75 Valve Body 2008-2012

$373.99View part

GM 6L80 6L90 6T70 Pressure Switch Seal Install Tool Kit

$22.99View part

First question: can this truck even set P0842?

Pressure switch codes are a Gen. 1 problem. Sonnax documents that Gen. 2 TEHCMs eliminated the pressure switches and use "clutch pulse learning" instead. The TCM pulses the clutches roughly every 1,250 miles to track shift timing and wear. Sonnax puts the change at the 2013 model year for the 6T70 series, and warns that it did not happen on every model at once, so you check the RPO code rather than assume it from the year.

On the Lambdas, the published bulletins line up like this:

To identify the generation, Sonnax says to find the transmission RPO on the Service Parts Identification label in the glove box, or to read the middle number under the barcode on the TCM cover. If a late Lambda reports pressure switch codes, check that the TEHCM is the right generation before anything else. Sonnax warns that a Gen. 2 module will not work properly in a Gen. 1 unit, and that ordering by the part number on the old module can bring you the wrong one.

What switch 1 does on the 6T75

Per GM's circuit description for P0842/P0843, the switch lives inside the control solenoid valve assembly (GM's Q8), which GM says has no serviceable components. The TCM supplies the signal and the switch grounds through the case. It is normally closed, which reads LOW at 0 volts. Oil pressure opens it and the signal goes HIGH at 12 volts.

Pressure control solenoid 2, a normally high solenoid, runs the 3-5-R clutch regulator valve. With the 3-5-R clutch released, the regulator valve rests in a position that feeds oil to the switch, so the switch reads HIGH. When the clutch is applied, the valve strokes and exhausts the switch circuit, so it reads LOW. Sonnax describes the same layout for all four Gen. 1 switches: each one is tied to a clutch regulator valve, and the TCM uses them to confirm gear state and to time clutch apply for adaptive learning.

GM's switch logic table gives the states. For switch 1 the pattern is short:

Switch 1 should go LOW only in Reverse, 3rd and 5th. GM's table for this application lists switches 1, 3, 4 and 5 and says outright that TFP switch 2 does not exist. Some aftermarket training numbers the four Gen. 1 switches 1 to 4 and calls the 3-5-R switch "switch 2." Go by the GM DTC definition, which ties P0842 to the 3-5-R regulator valve.

What the TCM is actually testing

Per GM, the P0842 monitor runs only when none of the fluid temp, input speed, output speed, solenoid or TEHCM codes are set (P0711-P0713, P0716, P0717, P0722, P0723, P0742, P0751, P0756, P0757, P0973, P0974, P0976, P0977, P1915, P182E), the high side driver is on, the engine has been at 400 RPM or more for 5 seconds and is at 550 RPM or more, ignition voltage is at least 8.6 volts, and fluid temperature is between -7 and 120°C.

It counts one fault each time switch 1 stays LOW after the 3-5-R clutch moves from a high pressure state to a low pressure state. In other words, the clutch released, the valve should have come back and pressurized the switch, and the switch still reads closed. P0842 sets at 25 counts. The opposite fault, P0843 (stays HIGH after apply), sets at 35. A short to ground sets P0842. An open, high resistance or short to voltage sets P0843.

P0842 is a Type C DTC. The TCM's response is to freeze 3-5-R clutch adaptive learning. No limp mode, no warning lamp guaranteed. GM's diagnostic aid notes that normal debris in the fluid can make a TFP switch set intermittently, and that a bad switch usually causes no symptom unless it fails continuously for a long time. On a high-mileage Lambda with frozen 3-5-R adapts, the first thing the owner usually notices is the 2-3 or 4-5 shift getting worse.

Four ways switch 1 ends up stuck LOW

1. Signal shorted to ground inside the TEHCM. The whole circuit is in the module, so this means the module itself, its connector pins, or metallic debris bridging terminals.
2. A failed switch. Sonnax reports "waffled or delaminated" switch films at teardown. High pressure or debris breaks the film seal, and contamination gets to the piston and contacts.
3. No oil reaching the switch on release. A 3-5-R regulator valve that hangs stroked, or a bore that leaks, never pressurizes the switch. Sonnax ties wear in this bore to valve oscillation and lists the related symptoms: 2-3 flare, 4-5 flare, delayed Reverse, Reverse slip and ratio codes.
4. Debris or a damaged filter plate. GM's TEHCM inspection specifically checks the four filter plate retaining tabs for cracks and tension. Material from a 3-5-R failure packs into that plate.

On 2007-2009 Lambdas, number four is the one to watch. GM bulletin 09-07-30-012F covers a broken 35R waved plate on 6T70/6T75 units. P0776 is the headline code, and P0777, P2723, P0716, P0717, P2714 and P2715 can set as well. GM says the failure normally does not occur before about 20,000 miles, and that the debris can affect the speed sensors and the other clutches. A switch code in the same circuit, on a unit that is already making metal, is a mechanical problem, not a wiring one.

Diagnosis on the Lambda, step by step

1. Read all the codes and do the others first. GM's procedure puts every speed sensor, temp, solenoid and P0776 code ahead of P0842. If P0776 or P2723 is stored with it, assume a 3-5-R mechanical fault until you prove otherwise.

2. Check the fluid. The 6T75 has no pan to drop. You are drawing a sample from the drain plug or the fill tube. Bright metal or friction material means teardown, not module replacement.

3. Watch the switches live. GM's verification: engine idling in Park, parking brake on, wheels chocked. Put the TFP Switch 1, 3, 4 and 5 parameters on the scan tool. Shift Park, Reverse, Neutral and compare against the table. Then go to Drive and command each forward gear. Switch 1 should read High, drop Low in Reverse, come back High in Neutral, and go Low only in 3rd and 5th.

4. If it follows the table, treat it as intermittent. Drive it under the enable conditions or recreate the freeze frame. GM states that if the vehicle passes verification, you do not continue to circuit and component testing, because that is how parts get replaced for nothing.

5. If switch 1 stays Low, bench the module. Ignition off, remove the Q8 assembly and inspect it per GM: connector pins for bends, debris or a broken retaining tab; no metal near the terminals; solenoid leads clean; filter plate tabs intact and tight. Set it on a clean surface beside the transmission, connect it to the vehicle harness through the DT-47825-20 adapter harness at the X1 20-way connector, ignition on, and gently press each TFP switch with a pencil eraser. Each parameter should change state. If switch 1 does not, GM's call is to replace the assembly.

If switch 1 toggles on the bench, the module is fine and the oil is not getting there. That puts you in the valve body at the 3-5-R regulator valve.

The fix, by cause

Failed switch film. This used to mean a new TEHCM. Sonnax now sells a pressure switch rebuild kit, 124740-70K, listed for Gen. 1 6T70/6T75 as well as 6L45 through 6L90, with an installation tool kit, 124740-TL70. Rebuilding the switches on a module that is already programmed to the vehicle saves you a module and a programming session.

Bad TEHCM. Replace it with the correct generation, matched by RPO, year and model, and program it. Do not wash the old one in solvent to "clean" it. GM's 09-07-30-012F says not to, and also says not to replace the assembly for any DTC unless SI diagnostics call for it.

Worn 3-5-R regulator bore. This is where Lambda model year matters. Sonnax's Gen. 1 chart shows 2007 and 2008 units with a 1.35 ratio 3-5-R regulator valve, and 2009 through 2013 units with a 1.83 ratio valve. The oversize kits follow that split: 124740-21K for the 1.83 ratio valve (.335 in. small spool) and 124740-26K for the 1.35 ratio valve (.390 in. spool). A 2007 Outlook and a 2011 Traverse do not take the same kit. Measure the spool before ordering. A vacuum test on the bore tells you whether it needs the ream at all.

Broken waved plate. Follow 09-07-30-012F. The updated 3-5-R waved plate is P/N 24254103, and GM recommends installing a new one any time the unit is apart. Debris has to be cleaned out of the whole transmission, which means splitting the case halves, and the pump fluid filter and the control solenoid filter plate must be replaced. GM warns that skipping those steps can lead to repeat damage.

After any repair: reset adapts. GM requires an adapt reset after internal repairs, valve body work or control solenoid valve assembly replacement. Clear codes, ignition off and wait up to 2 minutes for full power-down, then run the engine and reset adapts with the scan tool. Road test, then ignition off for 2 minutes. On the later Gen. 2 Lambdas, PIP5363B adds a learn drive for 2-6 adapts: M5, 38 mph or more, fluid above 30°C, about 5 miles of steady throttle on flat ground. It is a useful pattern for any light-throttle shift complaint after the reset.

FAQ

Can a 2015 or newer Traverse, Acadia or Enclave set P0842?

It should not set it as a pressure switch code. Sonnax documents that Gen. 2 TEHCMs eliminated the pressure switches and use clutch pulse learning, with the change starting in the 2013 model year for the 6T70 series. GM's 2015-2017 Lambda bulletin PIP5363B deals with unlearned adapts and no DTCs instead. If a late unit reports switch codes, verify which generation of TEHCM is installed.

Is it safe to drive a 6T75 with P0842?

Usually it drives normally. GM classifies P0842 as a Type C DTC, and the TCM response is to freeze 3-5-R clutch adaptive learning. If it is stored with P0776 or P2723, or the vehicle has lost Reverse, 3rd or 5th, stop driving it and check the fluid for debris, because that points to a broken 3-5-R waved plate.

Will a new TEHCM fix P0842 on a 6T75?

Only if switch 1 or its circuit inside the module is bad. GM's test is to bench the module on the DT-47825-20 adapter harness and press each switch with a pencil eraser. If switch 1 changes state, the module is fine and the problem is oil not reaching the switch at the 3-5-R regulator valve.

What is the difference between P0842 and P0843?

P0842 means switch 1 stayed LOW (0 volts) after the 3-5-R clutch released, and it sets at 25 counts. P0843 means it stayed HIGH (12 volts) after the clutch applied, and it sets at 35 counts. A short to ground sets P0842, while an open, high resistance or short to voltage sets P0843.