U0020: Low Speed CAN Communication Bus Performance
What to do
Everything probably works, and there will be no symptom until it stops. That combination makes this the quietest fault in this whole range.
Two things are happening at once. The network is correcting errors rather than simply carrying messages — which it is good enough at to hide completely. And this particular network is designed to keep running on one wire if the other fails, which hides the rest.
So "nothing seems wrong with it" is not evidence, and neither is a clear test drive.
There is a specific and reasonable thing to ask a garage for, and it turns this from an opinion into a measurement: ask them to read the network's error counters, and whether it is running in single-wire mode. A proper diagnostic tool reports both; a basic code reader shows neither.
If it is running on one wire, the reserve has already been spent and the next fault is the one that stops things — so the cheap moment to fix it is now.
Mention damp carpets, water in the boot, and anything fitted to the car after it was built — a tracker, dashcam, alarm or stereo, all of which can load a network they were never designed into.
What this code means
The low speed data bus is working, and not working properly. See U0011 for what a performance code means — an error rate rather than a failure — and U0019 for why this particular network hides its faults.
Those two arguments compound here, and that is worth stating plainly because it makes this the quietest serious code in the range.
A performance code means the bus is spending part of its time correcting itself rather than carrying traffic. And a fault-tolerant low speed network is already designed to carry on through damage. So a vehicle can be running on one wire, correcting errors on it, and presenting a driver with absolutely nothing to notice.
There is no symptom to judge by, and there will not be one until it stops.
Which makes the measurement the only honest guide, and it needs the manufacturer's figures rather than the familiar 60 ohms — see U0019. What a technician should be asking for is the network's error counters and whether it is running in single-wire mode, both of which a proper scan tool reports and neither of which a code reader shows.
That is a specific and reasonable thing to ask for, and it turns "nothing seems wrong" into a measured answer.
Causes are the marginal ones from U0011: a connector corroding but still conducting, a conductor down to a few strands, damp in a joint, or an added device loading a network it was never designed into.
Likely causes
Start here
- Corroded or loose connector · easy
- Poor wiring splice in an aftermarket installation · moderate
- Damaged data bus wiring · moderate
- Water ingress into connector · moderate
Less common
- Intermittent wiring fault · moderate
- Failed bus terminating resistor · specialist
Symptoms
Components involved
Information, not instruction
CarGeek is a reference, not a service manual. Its pages are written from public sources and from what other people have found, never from a manufacturer's own service information, so they can be wrong or incomplete for your particular vehicle. Some of what they describe carries real risk: high voltage, fuel under pressure, stored energy, hot and moving parts.
Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Community experience
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What makes one worth reading:
- The car — make, model, year and engine. The same fault behaves differently on each.
- What you measured, not only what you replaced.
- What actually fixed it — or that nothing did, which is worth knowing too.
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