Code
DF993
RENAULT
D
-> P1645 - relay control heater 2
AI status
Completed
Completed
100%
Causes
- Blown fuse for heater relay circuit
- Failed or sticking Heater Relay 2
- Open/short in wiring between ECU, relay and heater
- Corroded/loose relay socket or connector pins
- Faulty heater element (overload/short)
- Poor ground connection
Symptoms
- Malfunction Indicator Lamp (MIL) or ECU fault light on
- Heater(s) not heating or reduced heater function (cold start issues)
- Longer cranking or harder cold starts
- Heater circuit not activating when commanded
- DTC stored and possibly pending
What to check
- Read and record all stored DTCs and freeze-frame data with a diagnostic scanner
- Visually inspect fuse(s) related to heater circuits and replace if blown
- Inspect Heater Relay 2 and its socket for corrosion, heat damage, or loose pins
- Swap relay with identical known-good relay (if available) to confirm relay behavior
- Check for voltage at relay supply terminal (battery voltage present)
- Command the heater relay ON via scan tool and check for 12 V at the relay switched output while commanded
Signal parameters
- Relay coil/supply: battery voltage (~12 V) present at relay supply terminal at all times
- Relay activation: ~12 V present at relay switched output only when ECU commands heater ON
- Relay coil resistance: expected to be in the low-to-moderate ohm/ohm-to-hundreds of ohms range — consult factory spec for exact value
- Heater element resistance: low ohms to tens of ohms (refer to service manual)
- Control signal behavior: steady DC voltage when commanded; may be pulsed on some systems (verify with oscilloscope or scan tool)
Diagnostic algorithm
- Connect a diagnostic scanner, read/record DF993 (P1645) and any related codes, note freeze-frame and conditions when fault set.
- Visually inspect relay 2, fuse(s), relay socket and wiring for obvious damage, corrosion or loose connections; repair as needed.
- Check and replace blown fuses. If a fuse was blown, inspect for short to power/ground before replacement.
- With ignition ON, verify battery voltage at relay supply terminal. If absent, trace supply fuse or wiring to battery.
- Command the heater relay ON using the scan tool while measuring voltage at the relay switched output and at the heater input. No voltage when commanded → suspect relay, relay driver, or wiring.
- Swap relay with an identical, known-good relay (if available) to determine if relay itself is faulty.
- Measure continuity/resistance from relay output to heater and from heater to ground; compare to factory specs. Very low resistance to ground indicates short; very high/open indicates open circuit.
- Inspect/repair connectors and grounds; clean corroded contacts and ensure secure mating.
- If wiring and relay test good but no control signal, investigate ECU output driver and vehicle communication lines. Follow manufacturer procedures for ECU testing/repair.
- After repair, clear codes, perform functional test (cold start if applicable), and re-scan to confirm code does not return.
Likely causes
- Failed heater relay
- Blown fuse or poor fuse contact
- Broken wiring or pinched harness to relay/heater
- Corroded connector at relay or heater
Fault status
Status
P1645 (DF993) - Relay control heater 2: circuit open/short or relay activation failure detected.
Repair difficulty: Medium
Diagnostic time: 0.5-2.0 hours
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