Troubleshooting
This troubleshooting guide applies to Symbio 700 unit controllers on Precedent and Large Foundation units with model number prefixes: Precedent - DHJ, DHK, DSJ, DSK, THJ, THK, TSJ, TSK, TZK, WHJ, WHK, WSJ, WSK, YHJ, YHK, YSJ, YSK, and YZK, and Large Foundation - EDK and GDK. Use it when diagnosing compressor 1 proving, compressor 2 proving, compressor proving trip, compressor proving lockout, and compressor contactor fail alarms. It covers compressor will not start, contactor not pulling in, proving circuit open, relay contact resistance, auxiliary switch checks, and the exact connector and wire paths used to verify the circuit.
Are you looking for this Symbio 700 Compressor Proving Circuit Troubleshooting for Precedent and Large Foundation? For units with ReliaTel controls (RTRM), see the ReliaTel Compressor Proving article, and for units with a Symbio 700 unit controller for Odyssey (TTA or TWA), see the Odyssey Compressor Proving article.
Symptoms and alarm codes
Use this section to match the alarm behavior to the likely fault.
- Alarm - Compressor Proving Trip Diagnostic – 15-minute lockout: The HPC or DTL opens while the compressor is running after the circuit has proved, and the Compressor Contactor loses its 24 VAC, causing the Aux Switch to open.
- Alarm - Compressor Proving Lockout Diagnostic – INSTANT UNIT LOCKOUT!: The Symbio Board calls for the Compressor Contactor, and the Contactor doesn't pull in within 7 seconds.
- Alarm - Compressor Proving Lockout Diagnostic – INSTANT UNIT LOCKOUT!: The Symbio Board calls for the Compressor Contactor; it pulls in and drops back out after 7 seconds.
- Alarm - Compressor Contactor Fail Diagnostic – INSTANT UNIT LOCKOUT!: The Compressor Contactor Aux Switch closes before it is supposed to.
Possible causes: HPC and/or DTL open, contactor coil open, wiring/connection error, no 24 VAC at the black wire on the Aux Switch, bad Aux Switch, Aux Switch not on the contactor correctly, technician manually pushes in the contactor, or the contactor is welded shut.
Abbreviation Names
| Abbreviation | Abbreviation Name |
|---|---|
| UC | Unit Controller (Symbio 700) |
| AB | Adapter Board |
| HPC | High-Pressure Cutout |
| DTL | Discharge Temperature Limit |
| CC | Contactor |
| TNS | Transformer |
| AUX | Auxiliary |
Interactive Troubleshooting
Symbio 700 Compressor Proving Troubleshooter
Precedent & Large Foundation — interactive companion to the Compressor Proving Circuit article
Compressor 1
Compressor 1: 24VAC Voltage Tests
Verify if 24VAC control power is present at the Unit Controller.
- Setup: Unit powered ON. Set your multimeter to VAC.
- Reference Point: Place your black meter lead on clean Chassis Ground.
Voltage Diagnostics Table
| Test Point | Wire Color | When should power be here? | Expected | What to do if it FAILS |
|---|---|---|---|---|
| UC-P8-4 | 🟠 Orange | Always (when unit power is ON) | 24VAC | No Voltage: Go to the UC-P8-4 Power Supply Path below to locate the break. |
| UC-P8-5 | 🟡 Yellow | Only during an active Normal Call or Service Test | 24VAC | No Voltage (LED 5 is ON): The board relay is failing to output power. Go to Relay Resistance Test. Note: You only have a 5-second window to test this. |
UC-P8-4 Power Supply Path
If UC-P8-4 has no voltage, track back through these connectors in order to find where the power is lost:
- AB-J20-4 (🟠 Orange wire)
- AB-J8-1 (🔵 Blue wire)
- PPM72 / PPF72-3 (🔵 Blue wire)
- PPF2 / PPM2-1 (🔵 Blue wire)
- TNS2 (Transformer 2)
Compressor 1: Resistance & Safety Circuit Tests
If you have 24VAC leaving the controller on UC-P8-5 but the contactor coil (CC1) refuses to pull in, check the safety limit switches and the contactor coil resistance.
- Setup: Unit powered OFF. Set your multimeter to Resistance (Ω).
Resistance Diagnostics Table
| Test Point 1 | Test Point 2 | Expected Resistance | What it means if it FAILS |
|---|---|---|---|
| UC-P8-5 (🟡 Yellow) | AB-J5-9 (🟠/⚫ Org/Blk) | ≤ 0.6 Ω | Higher than 0.6 Ω / O.L: A wire harness plug is loose/unseated, or a safety limit switch (DTL1 / HCP1) is tripped or failed open. Use the Run Circuit Paths below to trace. |
| AB-J5-9 (🟠/⚫ Org/Blk) | AB-J5-10 (🟡/⚫ Yel/Blk) | 5 to 12 Ω | O.L (Open Circuit): The contactor coil itself is burnt out, or there is a break in the coil wiring. Use the CC1 Coil Path below to isolate. |
Compressor 1 Run Circuit Paths
Revision C (Older Boards)
If you measure an open circuit (O.L.), trace these connection points in order with your meter leads to locate the break:
- UC-P8-5 (🟡 Yellow) to AB-J13-4 (🟠 Orange)
- DTL1 (🔴 Red wires) (Discharge Temperature Limit)
- AB-J13-3 (🔵 Blue) to AB-J13-2 (🔴 Red)
- HCP1 (🔴 Red wires) (High Pressure Control)
- AB-J13-1 (⚫ Black) to AB-J20-5 (🟡 Yellow)
Revision E (Newer Boards with Test Pins)
If your board has physical test pins, trace along this specific path:
Path: J20-4 > UC-J8-4 > UC-J8-5 > J20-5 > Test Pin Comp 1 Run > J13-1 > BK HPC1 > R HPC1 > J13-2 > Test Pin HPC1 > J13-3 > BL DTL1 > O DTL1 > J13-4 > Test Pin DLT1 > J5-9 > O/BK CC1 > Y/BK CC1 > J5-10
CC1 Coil Backtrace Path
If your coil resistance test is O.L., trace through these physical plugs:
- PPF81 / PPM81 (🟠/⚫ Pin 9 or 🟡/⚫ Pin 10)
- PPF23 (🟠/⚫ Pin 1) or PPM23 (⚫ Pin 1)
- CC1 Contactor Coil Terminals (⚫ Black wires)
Compressor 1: Auxiliary Contact Feedback Tests
These tests confirm that the contactor auxiliary switch successfully tells the Unit Controller the contactor has pulled in.
Revision C
Aux Voltage & Feedback Tests (Unit Powered ON)
-
Check Input Power:
- Measure VAC from UC-P8-2 (🔴 Red) to Chassis Ground.
- Expected: 24VAC.
- If no power: Check AB-J18-1 (sourced from UC-P3-1). If UC-P3-1 is dead, trace back: UC-J4-1 > PPF72/PPM72 > PPF1/PPM1 > TNS1 (Transformer 1).
-
Check Proving Output:
- When the CC1 contactor closes, the auxiliary contacts must close and send 24VAC back to UC-P8-3 (🔵 Blue).
Aux Resistance Test (Unit Powered OFF)
- Set meter to Resistance (Ω).
- Place one lead on UC-P8-3 (🔵 Blue) and the other lead on UC-P8-2 (🔴 Red).
- Manually push in the CC1 contactor plunger.
- Expected: ≤ 0.6 Ω. If higher or open, the auxiliary block contacts are dirty/failed and must be replaced.
Compressor 1 Auxiliary Contact Wiring Paths
Revision C Path
- Yellow Wire Leg: CC1 Aux N.O. (🟡 Y) > Plug PPF81/PPM81-5 (🟡 Y) > AB-J5-5 (🟡 Y) > AB-J20-2 (🔴 R) > UC-P8-2 (🔴 R)
- Brown Wire Leg: CC1 Aux N.O. (🟤 BR) > Plug PPF81/PPM81-6 (🟤 BR) > AB-J5-6 (🟤 BR) > AB-J20-3 (🔵 BL) > UC-P8-3 (🔵 BL)
Revision E Path (With Test Pins)
Path: UC-J8-2 > J20-2 > Test Pin TNS > J5-5 > Y/BK CC1 > O/BK CC1 > J5-6 > Test Pin Comp1 Prov > J20-3 > UC-J8-3
Compressor 2
Compressor 2: 24VAC Voltage Tests
Verify if 24VAC control power is present at the Unit Controller.
- Setup: Unit powered ON. Set your multimeter to VAC.
- Reference Point: Place your black meter lead on clean Chassis Ground.
Voltage Diagnostics Table
| Test Point | Wire Color | When should power be here? | Expected | What to do if it FAILS |
|---|---|---|---|---|
| UC-P9-4 | 🟡/⚫ Yel/Blk | Always (when unit power is ON) | 24VAC | No Voltage: Go to the UC-P9-4 Power Supply Path below to locate the break. |
| UC-P9-5 | 🟤 Brown | Only during an active Normal Call or Service Test | 24VAC | No Voltage (LED 15 is ON): The board relay is failing to output power. Go to Relay Resistance Test. Note: You only have a 5-second window to test this. |
UC-P9-4 Power Supply Path
If UC-P9-4 has no voltage, track back through these connectors in order to find where the power is lost:
- AB-J20-4 (🟡/⚫ Yellow/Black wire)
- AB-J8-1 (🔵 Blue wire)
- PPM72 / PPF72-3 (🔵 Blue wire)
- PPF2 / PPM2-1 (🔵 Blue wire)
- TNS2 (Transformer 2)
Compressor 2: Resistance & Safety Circuit Tests
If you have 24VAC leaving the controller on UC-P9-5 but the contactor coil (CC2) refuses to pull in, check the safety limit switches and the contactor coil resistance.
- Setup: Unit powered OFF. Set your multimeter to Resistance (Ω).
Revision C without Test Pins
Resistance Diagnostics Table
| Test Point 1 | Test Point 2 | Expected Resistance | What it means if it FAILS |
|---|---|---|---|
| UC-P9-5 (🟤 Brown) | AB-J5-11 (🟤/⚫ Brn/Blk) | ≤ 0.6 Ω | Higher than 0.6 Ω / O.L.: A wire harness plug is loose/unseated, or a safety limit switch (DTL2 / HCP2) is tripped or failed open. Use the Run Circuit Paths below to trace. |
| AB-J5-11 (🟤/⚫ Brn/Blk) | AB-J5-12 (⚫/🔴 Blk/Red) | 5 to 12 Ω | O.L (Open Circuit): The contactor coil itself is burnt out, or there is a break in the coil wiring. Use the CC2 Coil Path below to isolate. |
Compressor 2 Run Circuit Paths
Revision C (Older Boards)
If you measure an open circuit (O.L.), trace these connection points in order with your meter leads to locate the break:
- UC-P9-5 (🟤 Brown) to AB-J11-4 (🟠 Orange)
- DTL2 (🔴 Red wires) (Discharge Temperature Limit)
- AB-J11-3 (🔵 Blue) to AB-J11-2 (🔴 Red)
- HCP2 (🔴 Red wires) (High Pressure Control)
- AB-J11-1 (⚫ Black) to AB-J20-11 (🟤 Brown)
Revision E (Newer Boards with Test Pins)
If your board has physical test pins, trace along this specific path:
Path: J20-10 > UC-J9-4 > J20-11 > Test Pin Comp2 Run > J11-1 > BK HPC2 > R HPC2 > J11-2 > Test Pin HPC2 > J11-3 > BL DLT2 > R DLT2 > J11-4 > Test Pin DLT2 > J5-11 > BR/BK CC2 > BK/R CC2 > J5-12
CC2 Coil Backtrace Path
If your coil resistance test is O.L., trace through these physical plugs:
- PPF81 / PPM81 (🟤/⚫ Pin 9 or ⚫/🔴 Pin 10)
- PPF24 (🟤/⚫ Pin 1) or PPM23 (⚫ Pin 1 or 2)
- CC2 Contactor Coil Terminals (⚫ Black wires)
Compressor 2: Auxiliary Contact Feedback Tests
These tests prove whether the contactor auxiliary switch is successfully telling the Unit Controller that the contactor has pulled in.
Revision C without Test Pins
Aux Voltage & Feedback Tests (Unit Powered ON)
-
Check Input Power:
- Measure VAC from UC-P9-2 (🔵/⚫ Blue/Black) to Chassis Ground.
- Expected: 24VAC.
- If no power: Check AB-J18-1 (sourced from UC-P3-1). If UC-P3-1 is dead, trace back: UC-J4-1 > PPF72/PPM72 > PPF1/PPM1 > TNS1 (Transformer 1).
-
Check Proving Output:
- When the CC2 contactor closes, the auxiliary contacts must close and send 24VAC back to UC-P9-3 (🟠/⚫ Orange/Black).
Aux Resistance Test (Unit Powered OFF)
- Set meter to Resistance (Ω).
- Place one lead on UC-P9-3 (🟠/⚫ Orange/Black) and the other lead on UC-P9-2 (🔵/⚫ Blue/Black).
- Manually push in the CC2 contactor plunger.
- Expected: ≤ 0.6 Ω. If higher or open, the auxiliary block contacts are dirty/failed and must be replaced.
Compressor 2 Auxiliary Contact Wiring Paths
Revision C Path
- Red/Black Wire Leg: CC2 Aux N.O. (🔴/⚫ R/BK) > Plug PPF81/PPM81-7 (🔴/⚫ R/BK) > AB-J5-7 (🔴/⚫ R/BK) > AB-J20-8 (🔵/⚫ BL/BK) > UC-P9-2 (🔵/⚫ BL/BK)
- Brown Wire Leg: CC2 Aux N.O. (🔵/⚫ BL/BK) > Plug PPF81/PPM81-8 (🔵/⚫ BL/BK) > AB-J5-8 (🔵/⚫ BL/BK) > AB-J20-9 (🟠/⚫ O/BK) > UC-P9-3 (🟠/⚫ O/BK)
Revision E Path (With Test Pins)
Path: UC-J9-2 > J20-8 > Test Pin TNS > J5-7 > R/BK CC2 > BL/BK CC2 > J5-8 > Test Pin Comp2 Prov > J20-9 > UC-J9-3
Board Diagnostic: Relay Contact Resistance Check
If you suspect the Symbio 700 board's internal compressor relays are damaged (pitted or sticking), perform this manual test using your phone's service app.
Compressor 1 (CC1 Relay) Check Steps
-
Unplug the UC-J8 harness from the Red P8 connector on the Symbio board.
- Take note: Orange wire is UC-P8-4, Yellow wire is UC-P8-5.
- Connect to the board with the Symbio Mobile App. Go to Tools > Service Test Mode.
- Force initiate Cool 2 in the app to energize the relay. LED 5 on the board must turn ON.
-
Measure Resistance (Ω) directly across board pins P8-4 and P8-5.
- You only have a 5-second window to read this.
- Specification: Must be < 10 Ω. Repeat this test 3–4 times. If it ever reads above 10 ohms, replace the board.
Compressor 2 (CC2 Relay) Check Steps
-
Unplug the UC-J9 harness from the White UC-P9 connector on the Symbio board.
- Take note: Yellow/Black wire is UC-P9-4, Brown/Black wire is UC-P9-5.
- Connect to the board with the Symbio Mobile App. Go to Tools > Service Test Mode.
- Force initiate Cool 1 in the app to energize the relay. LED 15 on the board must turn ON.
-
Measure Resistance (Ω) directly across board pins P9-4 and P9-5.
- You only have a 5-second window to read this.
- Specification: Must be < 10 Ω. Repeat this test 3–4 times. If it ever reads above 10 ohms, replace the board.
Current Precedent and Foundation Adapter Board with Relay Protection and Test Pins.
WARNING:
Information in this article is intended for use by individuals possessing adequate backgrounds of electrical and mechanical experience and who comply with all federal, state, and local laws, rules, orders, or regulations related to the installation, service, or repair of a heating or central air conditioning product. Any attempt to install, service, or repair a heating or central air conditioning product may result in personal injury and/or property damage. The manufacturer or seller cannot be responsible for the interpretation of the information contained herein, nor can it assume any liability in connection with its use.