Chrysler 21 Oxygen Sensor Signal No Activity
How to read it
Code 21 reads as 2 flashes, pause, 1 flash on the check-engine light. Cycle the ignition key on, off, on, off, then leave it on with the engine off, and count the flashes on the power loss or check-engine light.
New to flash codes? The How to Read Chrysler OBD-I Codes with the Key Cycle walks through the test and what the numbers mean. The general OBD-I guide covers the rest.
What does Chrysler code 21 mean?
Oxygen Sensor Signal No Activity
Common causes of 21
- Aged or contaminated oxygen sensor with a lazy or dead signal
- Open sensor signal wire or corroded connector
- Exhaust leak near the sensor drawing in air
- Lean or rich mixture fault holding the sensor output still
- On some models a coolant temperature sensor fault sets this same code
How to fix 21
- Watch oxygen sensor voltage for cross counts on a scan tool
- Inspect the sensor wiring and connector for opens and corrosion
- Repair any exhaust leak ahead of the sensor first
- Test the coolant temperature sensor on models that share this code
- Replace the sensor only after wiring and leaks test good
- Clear the code and retest after a road test
Sources and review
Review: qualified. Read the source limits below.
- Chrysler OBD-I trouble codes, model and year page
live reference page (troublecodes.net)
Supports: code identity and Chrysler meaning
Fetched directly from the linked model page. Single publisher, so the record is qualified, not verified. Causes and check order are curator-written from the circuit the code names.
- Chrysler OBD-I trouble codes, model and year page
live reference page (troublecodes.net)
Supports: code identity and Chrysler meaning
Fetched directly from the linked model page. Single publisher, so the record is qualified, not verified. Causes and check order are curator-written from the circuit the code names.
Last updated 2026-10-10.
Report a correction for 21. Include your vehicle, the issue and a source if you have one.