Code
P1187
DS
P — Powertrain
Multifunction double relay control malfunction
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UK: 1
EN: 5
RU: 4
AI status
Completed
Completed
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Causes
- Faulty multifunction double relay (stuck open/closed or intermittent)
- Open, short or high-resistance wiring to relay coil or contacts
- Poor or corroded connector or terminal at relay or module
- Blown fuse supplying the relay or module
- Weak or missing ground at relay or ECU/BCM
- Control module (ECU/BCM) driver circuit fault or internal failure
Symptoms
- Related system(s) may not operate (fuel pump, fans, main power circuits) or operate intermittently
- Engine may crank but not start or may stall if fuel pump relay affected
- Cooling fan(s) may not run or run continuously
- Illuminated MIL / warning light stored with P1187
- Intermittent electrical faults or loss of accessories
What to check
- Retrieve freeze frame and complete DTC list with a capable scan tool; note related codes
- Visual inspection of relay, relay socket, associated wiring harness and connectors for damage, corrosion or heat signs
- Check relevant fuses and fusible links for continuity
- Verify good ground(s) at relay, chassis and control module
- Swap multifunction relay with known-good identical relay (if available and non-critical) to see if fault clears
- Backprobe/control-module output with DVOM or oscilloscope while reproducing fault to observe switching
Signal parameters
- Control output voltage from module to relay coil: normally 0 V (off) and ~12 V (on) or switched to ground depending on design
- Relay coil resistance: typically a few ohms to a few hundred ohms (check OEM spec)
- Supply voltage at relay common contact: ~Battery voltage (12 V) with ignition on
- Ground continuity: near 0 Ω between relay ground terminal and chassis
- Switching waveform (scope): clean transitions without excessive noise, bounce or dropouts
Diagnostic algorithm
- Read and record all DTCs and freeze frame data. Clear codes and attempt to reproduce the fault while monitoring.
- Identify which systems the multifunction double relay controls on this model (consult wiring diagram/service manual).
- Perform visual inspection of relay, socket and wiring for damage or corrosion; repair any obvious faults.
- Check and replace any related fuses. Confirm battery voltage at relay supply terminal with ignition ON.
- With ignition OFF, remove relay and measure coil resistance and continuity of contacts per spec. Compare to known-good relay if available.
- Reinstall and backprobe the relay control pin(s); operate the circuit with scan tool or by commanded activation while measuring control voltage and ground continuity.
- If control output from ECU/BCM does not switch correctly but wiring and relay are good, inspect for short to battery or open to ground in the harness.
- If intermittent, perform wiggle tests on harness and connector while monitoring signal; use scope to capture transient events if needed.
- Swap with identical, known-good relay (only if safe to do so) to confirm relay vs. module/harness cause.
- If wiring and relay test good but module output remains faulty, consider module repair/replacement and check for available software updates or TSBs before replacement.
- After repairs, clear codes and verify repairs by road test or repeated activation cycles to ensure fault does not return.
Likely causes
- Faulty or intermittent multifunction relay
- Damaged wiring harness to relay (chafing, pinched, rodent damage)
- Corroded/loose relay socket or connector pins
- Blown fuse feeding the relay or module output
- Ground connection loose or corroded at relay/ECU
Fault status
Status
Control circuit for multifunction double relay: abnormal operation detected (open/short/high resistance/intermittent). Fault may be in relay, wiring, connectors, fuse, ground or module driver output.
Repair difficulty: Medium
Diagnostic time: 1.0 - 2.5 hours
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