Reduction Agent Temperature Too High
The temperature of the diesel exhaust fluid (DEF) is exceeding specified limits.
Heads up: this code wasn't in our hand-written library yet, so DiagCoach AI built this page on the fly. Double-check critical specs against service info before condemning parts.
What it means (plain English)
The sensor measures the fluid temperature and if it gets too hot, the system triggers a fault code to prevent damaging components.
What the computer is actually seeing
The ECM monitors the DEF temperature sensor and if it reads above 150°C (302°F) for a certain duration, it triggers SPN 0429.
What a healthy reading looks like
DEF temperature should typically be between 0°C and 30°C (32°F to 86°F).
Guided diagnostic — the DiagCoach way
Don't just throw parts at it. Walk through these in order — each step tells you whether to keep going or stop and fix what you found.
- 11. Inspect the wiring and connector of the DEF temperature sensor for damage or corrosion. If damaged, repair or replace.
- 22. Use a scan tool to check the current DEF temperature reading. If it's reading above 150°C (302°F) without any apparent reason, proceed to step 3.
- 33. Inspect the DEF fluid for contamination or quality issues. Check if it's fresh and meets specifications. If it's contaminated, replace the DEF fluid.
- 44. If the readings are still off after checking the fluid and the sensor, consider replacing the DEF temperature sensor.
- 55. If replacing the sensor doesn’t resolve the code, check for any software updates for the ECM.
Common causes
- Faulty DEF temperature sensor
- Shorted wiring or connectors to the sensor
- DEF contamination or low quality
- Insulation issues causing excessive heat
- ECM software glitch
Typical repair cost
$$$
Related codes
Frequently asked questions
Is it safe to drive with this code?
It's not advisable to drive, as extended high temperatures can cause damage to exhaust components.
Will this code come back after clearing?
If the underlying issue isn't fixed, it is likely to return.
Why does this code appear sometimes with a DEF quality issue?
Poor DEF quality can lead to incorrect temperature readings, triggering this code.
Working a real vehicle right now?
Let DiagCoach walk you through it live with your specific symptoms, vehicle, and what you've already checked.
Start guided diagnostic →