P097A means the transmission control module (TCM) commanded shift solenoid A and detected an open circuit: the driver switched on and no current flowed. It belongs to a trio added when the SAE extended the shift solenoid codes with hexadecimal numbers: P097A for solenoid A, P097B for solenoid B and P097C for solenoid C, each meaning "control circuit open." The older codes for the same solenoid, P0973 and P0974, cover a short to ground or to power. An open is a different failure with a different list of causes, and the distinction matters because a shorted solenoid is usually a solenoid, while an open solenoid circuit is usually a wire.
Severity: moderate to high. Can you keep driving? Only carefully, and only if the transmission still shifts. The TCM treats a solenoid it cannot control as a solenoid it cannot trust, and most calibrations respond by restricting the available gears. On Allison transmissions that means a limited range or a hold in the current gear, with the CHECK TRANS light on. Driving a heavy vehicle in a restricted range for any distance overheats the fluid. Get it to a shop, or have it towed if the truck is loaded.
Which vehicles set P097A
Most passenger-car transmissions still use the older P0973 and P0974 numbers for shift solenoid A, so P097A shows up mainly on newer controllers that adopted the extended SAE list. The largest group is trucks, buses and motorhomes with Allison automatic transmissions on 5th and 6th Generation controls: the 1000 and 2000 series behind Duramax diesels in Chevrolet and GMC HD pickups and behind Cummins engines in Freightliner, International and Kenworth medium-duty trucks, and the 3000 and 4000 series in buses, refuse trucks and Class A motorhomes. Allison's code tables list the full set: P0973 and P0974 for shift solenoid 1 low and high, P097A for shift solenoid 1 open, P097B for shift solenoid 2 open, P097C for shift solenoid 3 open.
Allison numbers its solenoids rather than lettering them, so the code the scan tool calls "shift solenoid A" is shift solenoid 1 (SS1) on an Allison. SS1 is a normally closed on/off solenoid that sends control main pressure to shift valve 1, which the TCM strokes to reach the next range on an upshift or downshift.
How the circuit is wired, and why that locates the fault
On the Allison 1000 and 2000 series 5th Generation controls, the TCM feeds SS1 from high side driver 2 (HSD2) through wire 171, and wire 152 returns the circuit from the solenoid to its low side driver in the TCM. An open anywhere in that loop, on either wire, in the transmission internal harness, at the 20-way main connector on the side of the case, or in the solenoid coil itself, sets P097A.
The reason the other codes matter is that HSD2 is shared. It also feeds other solenoids, and Allison's troubleshooting note states the consequence directly: if P097A is stored together with P097B and/or P097C and/or P0964 (pressure control solenoid 2 open), the open is most likely in the high side of the circuit, that is, in wire 171 or in the driver feed, because one open there takes out every solenoid on that driver. P097A alone, with the companion solenoids reporting normally, means the open is on the low side (wire 152) or inside the SS1 coil, the only parts of the loop that SS1 does not share.
That single observation is the most useful thing on this page. Read every stored code before touching anything, and sort them into "shared driver" and "this solenoid only" before opening the harness.
Causes, in the order they are found
A connector problem at the transmission main connector or the TCM. Allison's own guidance for solenoid circuit codes starts with poor electrical connections at the TCM and at the transmission main connector: a terminal pushed back out of its cavity, a terminal spread by a probe, corrosion from a connector that sits low and takes road spray. The main connector on the side of the case is the single most common site because it sees heat, vibration and water at once.
A broken wire in the external harness. Between the TCM (mounted in the cab or on the frame, depending on the chassis builder) and the transmission, the harness crosses the frame rail and the bell housing. Where it is tied too tightly, rubs on a bracket, or was disturbed during an engine or exhaust repair, the copper breaks inside intact insulation. This is the classic intermittent P097A that sets on rough roads and clears on the highway.
A broken wire in the internal harness. Inside the transmission, the harness from the main connector to the solenoid body sits in hot fluid and vibrates with the valve body. Insulation hardens with age and cracks at the solenoid connector. An internal open is more common on high-mileage units and on transmissions that have run hot.
An open solenoid coil. The coil winding can burn open, particularly on a unit that has been overheated or that has run with contaminated fluid. Coil resistance measured at the solenoid connector tells the story: an Allison on/off shift solenoid should read a low, finite resistance (a few ohms to a few tens of ohms depending on the family), and an open coil reads infinite. Compare it against the neighboring shift solenoids on the same valve body.
The TCM driver. Last on the list. A driver that has failed open cannot power the solenoid, but a TCM only becomes a suspect after the harness has passed continuity and resistance tests end to end and the solenoid reads within specification.
Rodents and bad repairs. Two causes that belong on every open-circuit list: chewed insulation under the cab, and harnesses left unclipped or misrouted after a radiator, starter or exhaust job. Nesting material and droppings near the harness, or a fresh repair invoice, shorten the search.
What the TCM does when it sets
Allison controls respond to a solenoid circuit fault by inhibiting the shifts that depend on that solenoid and holding a range the transmission can reach without it. The CHECK TRANS light comes on, the selector may show a range mismatch, and the transmission stays in whatever range it can maintain. The pattern of available ranges is itself a diagnostic clue: with SS1 unavailable, certain upshifts and downshifts are blocked while others still occur, and the service manual's range inhibit table converts that pattern to the solenoid involved. Passenger-car controllers behave similarly, holding a fixed gear or a limited set of gears with the check engine light on.
A diagnostic order that finds the open before the solenoid is condemned
- Record every code and its status before clearing. Sort them: P097A alone, or P097A with P097B, P097C or P0964. That decides whether you start on the shared high side or on the SS1-only low side. Pull the freeze frame if the tool supports it on transmission codes; fluid temperature and road speed at the moment of the fault separate a hot-fluid internal open from a rough-road external one.
- Inspect both ends of the external harness. Disconnect the transmission main connector and the TCM connector, look at every terminal with a light, and check for pushed-back or spread terminals, corrosion and moisture. Reseat firmly and retest before anything else.
- Measure solenoid resistance at the main connector. With the connector disconnected, measure between the SS1 pins (on the 1000 and 2000 series, the wire 171 and wire 152 cavities). Infinite means an open in the internal harness or coil; a normal reading means the open is external. Compare with the other shift solenoids on the same connector.
- Check external harness continuity. With both connectors disconnected, measure wire 171 and wire 152 end to end. Each should be well under an ohm. Flex the harness at the frame crossing and near the bell housing while measuring to catch an intermittent break.
- Do not pierce the insulation. Allison warns specifically that piercing a wire to probe it creates a path for moisture and damages the wire, especially at the TCM. Back-probe at the connector or use the manufacturer's breakout box.
- If the internal circuit reads open, remove the pan and inspect. Look at the internal harness connector on the solenoid body, the harness routing and the solenoid connector. Measure the coil directly at the solenoid. Replace only what is measured open.
- Clear the code and verify with a full shift cycle, including several upshifts and downshifts through the range that SS1 controls. On Allison controls the code moves to history and clears itself after the calibrated number of warm-up cycles without recurrence; on a passenger car, drive until the readiness monitors complete.
What P097A does not tell you
It names the circuit that went open, not the point where it opened. It does not distinguish a coil from a wire from a connector, and it does not say whether the open is present now or was momentary. The companion codes narrow the location, the resistance and continuity measurements finish the job, and the solenoid comes last because it is the one component that can be tested by measurement rather than replaced on faith; the general principle is covered in what a trouble code does not tell you. A flashing check engine light with this code is a misfire, not the transmission; see flashing versus steady.
Frequently asked questions
Can I drive with a P097A code?
Only if the transmission still shifts, and only to reach a shop. The TCM restricts the available ranges when it cannot control a shift solenoid, and a heavy vehicle in a restricted range overheats its fluid quickly. A loaded truck should be towed.
Is P097A the same as P0973 or P0974?
No. P0973 and P0974 mean the shift solenoid A circuit is shorted low or high, and usually point at the solenoid. P097A means the circuit is open: no current flows when the driver turns on. That points at a broken wire, a bad connector or an open coil, in roughly that order.
Why do I have P097A together with P097B and P097C?
Because those solenoids share a high side driver and its feed wire. One open on the shared side takes all of them out at once. Allison's guidance is that P097A with P097B, P097C or P0964 means the open is most likely in the high side, which on the 1000 and 2000 series is wire 171 from high side driver 2.
How much does it cost to fix P097A?
A connector repair or an external harness splice is usually under an hour of labor. An internal harness or a single shift solenoid on an Allison requires dropping the pan and is a few hours plus fluid and filters. A TCM is the expensive outcome, and it comes last because the wiring and the solenoid can both be measured before the controller is blamed.
Related codes
- P0973 / P0974: shift solenoid A (Allison SS1) control circuit low and high. Shorts rather than opens.
- P097B / P097C: shift solenoid B and C control circuit open. Stored together with P097A when the shared high side is open.
- P0964: pressure control solenoid 2 control circuit open. Shares the driver with SS1 on Allison 1000 and 2000 controls, so it is part of the same high side pattern.
- P0700: transmission control system malfunction indicator request. Set by the TCM to turn on the engine's check engine light.
The code number itself tells you the family and the domain; how to read an OBD2 code number explains what each character means and why the A in P097A is a hexadecimal digit, not a letter.