P0697
PowertrainSensor Reference Voltage "C" Circuit Open
The ECM has lost the 5-volt reference supply it sends out to one bank of sensors, the 'C' circuit, and that circuit is reading open. The module gives several sensors their power from shared reference lines, so when one drops out you can lose readings from a whole group at once. For you that usually shows up as a warning light plus something the car does badly, rough running, dodgy gear changes, sometimes a no-start. It's an electrical fault at heart, not a worn-out part, so the fix often comes down to finding one bad connector or chafed wire rather than buying a big component.
ⓘ Information only. This page provides general educational information about fault code P0697. We do not assess the urgency or safety implications of any specific fault. That requires in-person diagnosis by a qualified mechanic. Full terms.
Recommended next steps
Whether a fault is urgent, drivable, or routine depends entirely on the cause on a specific vehicle, and that can only be determined by a qualified mechanic with diagnostic equipment. If a warning light is illuminated, the most reliable next step is professional diagnosis.
What does P0697 mean?
P0697 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Sensor Reference Voltage "C" Circuit Open.
This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.
Symptoms commonly associated with this code
Symptoms that drivers often report alongside this code. Not all may apply to every case:
- • Engine warning light on, sometimes flashing
- • Hesitation, lumpy idle, or stalling at junctions
- • Hard starting or a complete no-start on a bad day
- • Gearbox playing up, harsh shifts or refusing to change
- • Loss of drive mode selection on cars with sport/eco or 4WD switching
- • Worse fuel economy and a generally flat feeling under throttle
Possible causes
Causes commonly associated with P0697, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. Chafed, broken, or corroded wiring in the reference circuit, far and away the most common cause. Look where looms flex or rub against the engine or bulkhead
- 2. Corroded or backed-out connector pins, often where road salt and damp get in over a few UK winters
- 3. Poor earth or a tired earth strap pulling the whole supply down
- 4. A blown fuse or fusible link feeding that part of the harness
- 5. Weak battery or a failing alternator leaving the system short of volts, which can throw the reference reading out
- 6. A shorted sensor on the circuit dragging the 5V line down, MAP and accelerator pedal position sensors are usual suspects
- 7. Internal ECM fault, typically a failed voltage regulator inside the module. Real, but check everything outside the box first
How mechanics typically diagnose
A typical diagnostic sequence used by mechanics, provided here for educational reference only. Diagnostic work should be performed by a qualified mechanic with the appropriate tools and training.
- 1. Back-probe the reference pin at an affected sensor connector with a multimeter, key on. A healthy 5V circuit reads roughly 4.8 to 5.2V. Open or near zero confirms the supply is dead at that point
- 2. Pull the full code list and note the freeze-frame. P0697 rarely comes alone, and the sensor codes alongside it tell you which sensors share that 'C' line
- 3. Wiggle-test the harness and connectors while watching the live reference voltage. If it flickers when you move a section, you've found your fault
- 4. Check the related fuses and fusible links, and do it under load where you can. A fuse can look fine yet open up when the circuit pulls current
- 5. Test battery and charging voltage, you want over 13.5V running, and measure earth strap resistance, which should be under 0.1 ohm. A bad earth fools a lot of people into condemning the ECM
- 6. If wiring, earths and supply all check out and the reference is still open at the module connector, the ECM itself is the likely culprit. Confirm against manufacturer wiring data before spending
Common questions about P0697
What am I likely to pay to sort this out? +
It swings a lot depending on what's actually wrong. A wiring or connector repair at an independent garage is often £80-£200 once they've found it, and the finding is most of the labour. If it needs the ECM reprogramming, budget low-to-mid hundreds. A replacement module fitted and coded runs from several hundred at an independent up to four figures at a main dealer. Diagnostic time alone is usually £40-£90. Most cars never get that far because it turns out to be one dodgy connection.
How do I work out which cause it is on my own car? +
Start with the voltage at the sensor connector. If you've got a clean 5V there, the wiring to that point is fine and you're chasing a sensor or the next branch along. If it's dead or jumping about, work back toward the ECM, wiggling the loom while you watch the meter. Check earths and battery voltage before you even think about the module, because a tired earth or a flat-ish battery throws this code and gets blamed on far dearer parts. The freeze-frame and any partner codes point you at which sensor group is on the 'C' line.
Is this something I can fix myself? +
The diagnosis is doable for a confident DIYer with a decent multimeter, and if it turns out to be a corroded plug or a rubbed-through wire, repairing that is well within reach. Clean the pins, repair the wire properly with solder and heatshrink rather than crimp-and-pray, and seal it against the weather. What you can't safely DIY is reprogramming or replacing the ECM, that wants the right kit and is best left to a garage.
If I just clear the code, does it stay gone? +
Only if the fault was a one-off glitch, which is rare. Clear it and the light goes out, but an open circuit comes straight back the moment the car sees the same conditions, sometimes within minutes, sometimes after a cold damp morning. An intermittent connector might stay quiet for a while and fool you into thinking it's cured. Find the cause and fix it, otherwise you're just resetting the messenger.
Information only, not professional advice
The information on this page is provided for general guidance and educational purposes only. It is not a substitute for diagnosis or repair advice from a qualified mechanic. Always verify any fault before paying for repairs. carfaultcodes.co.uk accepts no liability for decisions made based on this information. Full terms →