What this code means
This code may indicate that the PCM is seeing a high signal from the transmission fluid pressure switch D circuit for the 5th clutch.
What the vehicle may do
- The transmission may operate in a fail-safe strategy.
- Shift operation may be limited while the code is stored.
- The concern may be intermittent if the signal returns to normal during testing.
Possible fault areas
- A possible pressure switch issue.
- Possible wiring or terminal problems between the switch and the PCM.
- Possible PCM-side circuit concern after the switch and wiring have been checked.
Diagnostic path
Open and frame the fault
On this Accord, P0878 points to the transmission fluid pressure switch D circuit for the 5th clutch reading high. In plain terms, the PCM may not be seeing the 5th clutch pressure switch signal the way it expects. The car may go into a fail-safe strategy, and with this code stored it does not shift to any gear other than 2nd through 5th gear. Broadly, this can involve the pressure switch, the switch circuit between the transmission and the PCM, terminal fit or connection problems, or in some cases the PCM side of the circuit. Start with the basic system checks and follow a structured diagnostic approach before getting into the pinpoint test.
Verify the problem first
Begin the verification with the vehicle in ON mode, then clear the code with the HDS. Raise the vehicle on a lift, make sure it is securely supported, and make sure all four wheels can rotate freely. Start the engine, then turn VSA off with the VSA OFF button. If vehicle speed will be above 31 mph (50 km/h), enter VSA maintenance mode.
Watch the 5th pressure switch signal
Now run the vehicle in 5th gear and watch the 5th Pressure Switch parameter on the HDS. The expected threshold for that signal is ON in 5th gear. Then slow down and stop the wheels. If the signal matches the ON threshold in 5th gear, treat it as an intermittent failure and the system is OK at this time. Check for poor connections and loose terminals between the 5th clutch pressure switch and the PCM. If captured operating data is available for this code, try to reproduce those same conditions. If the signal does not match the threshold, the failure is duplicated, so continue into circuit isolation.
Isolate the switch from the rest of the circuit
With the failure duplicated, turn the vehicle to OFF LOCK mode. Disconnect the transmission fluid pressure switch D, 5th clutch, 1P connector. Then use a jumper wire from terminal No. 1, the light blue wire at that connector, to body ground. Turn the vehicle back to ON mode and check the 5th Pressure Switch parameter again with the HDS. The threshold is still ON. If the parameter now matches ON, the circuit responds when grounded, so replace transmission fluid pressure switch D, 5th clutch. If the parameter still does not match ON, leave the switch out of the decision and move to the OP5SW open-wire check.
Check the OP5SW wire
For the OP5SW line check, turn the vehicle to OFF LOCK mode and remove the jumper wire. Jump the SCS line with the HDS, then disconnect PCM connector B, the 51P connector. Check continuity on the OP5SW circuit between the 5th clutch pressure switch 1P connector No. 1, light blue wire, and PCM connector B, 51P, terminal No. 27, also light blue. If there is continuity, the OP5SW wire is OK, and the next supported repair is to replace the PCM. If there is no continuity, repair the open in the OP5SW wire between transmission fluid pressure switch D, 5th clutch, and the PCM.
Takeaway
The key on P0878 is to prove whether the PCM can see the 5th clutch pressure switch circuit go ON when the switch circuit is grounded. If it can, the switch is the failure area. If it cannot, prove the OP5SW wire before condemning the PCM. For more diagnostic training, visit stepdiagnostics.com.
Final check
This diagnosis is about separating a pressure switch problem from an open circuit or PCM-side issue by watching the switch signal and then checking the OP5SW circuit.
For more guided automotive diagnostics, visit STEP Diagnostics.
