P0166: Downstream oxygen sensor on bank 2 (sensor 3) is sending no signal activity to the engine computer
O2 sensor circuit no activity (bank 2 sensor 3)
Severity: Inspect soon
TroubleCodes.net lists power loss, hesitation, higher fuel use and possible engine harm from a rich mixture for P0166, so get it checked soon.
P0166 in short
P0166 means the engine computer sees no signal activity from the oxygen sensor labelled bank 2, sensor 3, a downstream sensor after the catalytic converter on the side without cylinder 1. Damaged wiring, a blown heater fuse or a failed sensor are the usual causes. Get it checked soon.
The computer expects the oxygen sensor voltage to move up and down as exhaust changes. P0166 is set when that sensor shows no activity at all.
Bank 2 is the cylinder bank that does not hold cylinder 1. TroubleCodes.net says sensor 3 is the downstream sensor after the catalytic converter, and that vacuum or exhaust leaks and other stored codes should be dealt with first.
Most likely causes
These are general causes for this code across vehicles. None is specific to one make or model.
Cause 1: Damaged wiring or connector in the signal or heater circuit
Common general cause- Why it ranks here
- TroubleCodes.net lists damaged signal and heater circuit wiring first, and a broken wire gives no signal at all.
- What would change this
- Rises if the harness is burnt, chafed or the connector is corroded. Falls if wiring and connector are intact.
Cause 2: Blown fuse in the heater control circuit
Common general cause- Why it ranks here
- TroubleCodes.net says a blown fuse can stop the element heating, so the sensor stays inactive.
- What would change this
- Rises if the heater fuse is open. Falls if the fuse is good.
Cause 3: Defective oxygen sensor
Common general cause- Why it ranks here
- TroubleCodes.net names a defective sensor, which is common once wiring and fuses check out.
- What would change this
- Rises if wiring and power are good but the sensor shows no activity. Falls if the sensor signal moves normally.
Cause 4: Exhaust or vacuum leak, or silicone additives
Common general cause- Why it ranks here
- TroubleCodes.net lists silicone-based oil or fuel additives and internal engine problems as non-electrical causes.
- What would change this
- Rises if a leak is found or additives were used. Falls if the engine runs cleanly with no leaks.
Cause 5: Engine computer fault
Possible — needs testing- Why it ranks here
- TroubleCodes.net calls PCM failure rare, so it comes last.
- What would change this
- Rises only after wiring, fuses and sensor test good. Falls if any other cause is found.
What to test first
Start at step 1. Each result tells you what it means and where to go next.
Step 1: Read all stored codes and freeze-frame data, and note any other codes. Resolve those and any exhaust or vacuum leaks first.
IfOther engine or oxygen sensor codes are stored
It meansDeal with those first, since they can affect this one.
IfOnly P0166 is stored
It meansMove on to the sensor wiring.
Step 2: Look over the wiring and connector of the bank 2 downstream sensor for burns, chafing or corrosion, and check the heater fuse.
Warning:Exhaust parts stay hot long after the engine stops; let it cool fully and keep wiring away from the exhaust when you reroute it.
IfDamaged wiring or an open fuse is found
It meansA circuit fault is likely.
Points to: Damaged wiring or connector in the signal or heater circuit
IfWiring and fuse look fine
It meansTest the circuit values and the sensor.
Step 3: Check resistance, voltage, ground and continuity on the signal and heater circuits against the maker's specification, then remove the sensor and look for discoloration or deposits.
Warning:Back-probe connectors gently and avoid shorting pins, which can damage the sensor circuit or computer; use the correct socket so the sensor and exhaust threads are not damaged.
IfCircuit values are out of specification
It meansRepair the circuit.
Points to: Damaged wiring or connector in the signal or heater circuit
IfCircuit is good but the sensor is discolored, fouled or inactive
It meansThe sensor is suspect.
Symptoms
- Check engine light on
- Loss of power or hesitation
- Higher fuel use or black exhaust smoke
- Hard starting
How to fix
Step 1: With the engine off and cool, look over the visible sensor wiring and connector for damage and check the related fuse.
Parts and cost context
- Oxygen (O2) sensor:Replace only after wiring and fuses test good. Labour varies; get a quote.
Labor varies by vehicle and shop; get a quote before you approve a repair.
Compare used part prices on PartPriceGuideOn your vehicle
We don't have reviewed vehicle-specific evidence for P0166 yet. The causes and tests above apply across vehicles.
Related codes
No curated relationships for P0166 yet.
P0166 questions
Is it safe to drive with P0166?
TroubleCodes.net lists power loss, hesitation and higher fuel use, so have it checked soon rather than ignoring it.
Sources and review
- Last reviewed
- Written by
- Saurabh K.
- Editorial review
- Pending
- Technician review
- Technician review pending
This page explains what a code usually means; it is not a substitute for a professional inspection. How we source and review · Report an error