OBD-II trouble code
P0690: ECM/PCM Power Relay Sense Circuit High
The module's feedback input is reading higher than it should. Two very different faults produce that — genuine overvoltage from the charging system, or voltage present at a moment when the relay should have released — and they call for opposite repairs.
Quick facts
- System
- Powertrain
- Category
- PCM / Electronics
- Severity
- High severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $30 – $1,200
- DIY difficulty
- Advanced DIY
What does P0690 mean?
This is the one code in the power relay block where high can mean two entirely different things, and getting the order of investigation right saves both money and hardware.
The first possibility is that the voltage really is too high. The sense circuit reads the relay's output, and the relay's output is system voltage, so a charging system pushing sixteen volts or more presents itself here as an out-of-range reading. That is not a relay fault at all — it is a failing voltage regulator or alternator, and it is the version of P0690 that does damage while you think about it. Sustained overvoltage cooks electrolytic capacitors inside every module on the vehicle, boils electrolyte out of a lead-acid battery, and shortens the life of every bulb. It is also the fastest thing on this page to rule in or out: put a meter across the battery with the engine running, and if the number starts with a sixteen you have your answer in under a minute and should stop driving until it is fixed.
The second possibility is that the voltage is normal but present at the wrong time. The module commands the relay off, then reads its sense input expecting that voltage to disappear. If it does not, the relay's contacts have welded shut, or the sense wire is shorted to a permanently live feed. Either way the module concludes it is seeing voltage it did not ask for.
A welded relay leaves the vehicle powered after key-off, which drains the battery overnight and produces a maddening pattern: flat battery, jump start, drives fine all day, flat again the next morning.
And a welded relay carries a lesson that is easy to skip. Contacts weld because too much current passed through them, so the relay is a symptom and not the cause. Fit a new relay without finding the shorted or overloaded circuit that welded the old one, and the new one welds too. Before condemning the relay, measure what its output circuit actually draws and account for it.
The two mechanisms also explain why this page is rated the way it is. The welded-relay version drives fine and strands you in the morning. The overvoltage version drives fine right up until it has quietly damaged several modules. Neither will stall you in traffic, but one of them is doing continuous harm — which is why checking charging voltage first is not a suggestion here, it is the order of operations.
Common causes
- Failing voltage regulator or alternator producing genuine system overvoltage
- Welded main relay contacts holding voltage on after the module commands the relay off
- Sense wire shorted to a permanently live circuit through damaged insulation
- Corrosion in the underhood fuse box bridging the sense terminal to an adjacent power feed
- Overloaded or shorted circuit downstream of the relay that welded the contacts
- Failed sense input inside the PCM, which is uncommon and should be proven last
Symptoms
- Battery flat after sitting overnight, repeatedly, with no obvious accessory left on
- Cooling fan, fuel pump or relay still running after the key is removed
- Headlights unusually bright, or repeated bulb failures, when overvoltage is the cause
- Battery case swollen, hot, or venting a sulphur smell after overcharging
- Check engine light with P0690 stored, often alongside other charging or module codes
- Engine drives and performs normally, which makes the fault easy to postpone
Diagnostic steps
- 1.Measure battery voltage with the engine running before anything else. A reading meaningfully above roughly 15 volts means overvoltage, and that is a charging system repair, not a relay repair.
- 2.If charging voltage is normal, shift to the timing question: does voltage remain on the sense circuit after the module should have shut the relay down?
- 3.After key-off and the normal module sleep interval, listen and feel for the main relay still being energised, and perform a parasitic draw test.
- 4.Remove the relay and check whether the sense circuit still shows voltage. Voltage with the relay out means the wire is shorted to a live feed rather than the relay being welded.
- 5.Inspect the relay's contacts and socket for the discoloration and pitting that accompany welding, and check the underhood fuse box for corrosion and water intrusion.
- 6.If the contacts are welded, measure the current drawn by the circuit the relay feeds. Something overloaded them and it will do it again to a new relay.
- 7.Confirm the repair by verifying charging voltage in range and the relay releasing cleanly after key-off, then rechecking the parasitic draw.
Repair cost
$30 – $1,200
The two causes have very different bills. A welded relay is $15-$60 for the part, plus whatever caused the overload. Alternator or voltage regulator replacement runs $400-$900 and is the more expensive branch. Add a battery at $120-$250 if it has been overcharged or repeatedly discharged, since neither treatment is survivable. If overvoltage has damaged a module, that cost sits on top.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with engine main / powertrain relay replacement preselected. Adjust labor rate and vehicle category to fit your situation.
DIY vs shop
This is an advanced DIY job. It typically requires specialty tools, scan-tool access, lifting equipment, or careful sequencing to avoid causing new failures. Plan for extended downtime and have a backup vehicle. Most owners are better served by a shop that has done this repair before.