U04A4 code meaning
Invalid Data Received From Particulate Matter Sensor A
Network system Generic code U04xx codes Check engine light
What U04A4 means
Diesel particulate matter (PM) sensors have their own control unit that measures soot on the element and sends the result to the engine controller. GM's 2025 ECM tables (25OBDG04A) check the alive rolling count, protection value and checksum of about twenty soot sensor signals and set U04A4 after 3 bad frames in 10, with battery voltage of at least 11 V for 5 s; it is a Type B code, so the MIL comes on after two trips. VW's 2014 Touareg TDI chart sets it when the sensitivity factor leaves 0.75 to 1.25, when an invalid sensitivity signal lasts 2 s or when sensor messages fail for 0.6 s. Ford's 2014 chart also lists it as a MIL code. U02A3 is total loss, U04B2 the second sensor, U05A4 the reverse direction. The filter monitor pauses until data are valid.
U04A4 at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modeNot expected
- Repair difficultyMedium
- Typical labor0.5 to 2 hours
- Clears after repairAfter a drive cycle
Symptoms of U04A4
- Check engine light on
- Particulate filter monitor not completing, readiness missing
- Usually no change in how the engine runs
Common causes of U04A4
- Most likelyPM sensor control unit or sensing element faulty (heat, soot, moisture)
- PossibleReplacement sensor of the wrong variant or not yet matched
- PossibleDamaged wiring or connector between sensor unit and engine controller
- Less likelyLow supply voltage disturbing the sensor unit
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Pm sensor a
What each part of U04A4 means
| U | Network system: communication between the control modules (CAN bus). |
| 0 | Generic code: it means the same on every make (SAE standard). |
| A4 | The exact fault in that area. |
What to do next
- Note U04A4 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about U04A4
What does U04A4 mean?
U04A4 means "Invalid Data Received From Particulate Matter Sensor A". Diesel particulate matter (PM) sensors have their own control unit that measures soot on the element and sends the result to the engine controller. It is a generic code, so it means the same on every make.
Is it safe to drive with U04A4?
Usually yes, for a short time, if the check engine light is steady and the car drives normally. If the light flashes or the car runs badly, stop and have it checked.
Will U04A4 fail an emissions test?
Yes, it can. U04A4 is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is U04A4 to fix?
Difficulty: medium. A typical repair takes 0.5 to 2 hours of work. This is a job for a workshop.
Will U04A4 clear itself after the repair?
After the repair the car has to complete a drive cycle (a mix of city and highway driving) before the test runs again. Until then the readiness monitor shows "not ready", which can fail an inspection.