Honda P1166 Code: A/F Sensor (Sensor 1) Heater System Electrical
The circuit that powers the heater in Honda's upstream A/F sensor is open, shorted or unpowered.
At a glance
Severity: Low
- Meaning
- A/F Sensor (Sensor 1) Heater System Electrical
- System
- Fuel & Air Metering · Powertrain
- Safe to drive?
- Yes, short term
- Typical repair cost
- $1–$550
- DIY difficulty
- Easy
A wideband A/F sensor only reads correctly when its tip is several hundred degrees hot, so it contains an electric heater that draws far more current than an ordinary oxygen sensor heater. On many Hondas that heater is fed through its own fuse, and on several models, such as the early-2000s Accord and CR-V, through a dedicated A/F sensor heater relay as well, with the PCM controlling the circuit. P1166 is the electrical half of the heater diagnosis: the PCM saw a voltage on the heater control line that doesn't fit its command, which means an open element, a blown fuse, a relay that doesn't click in, or a broken or shorted wire. It very often appears together with P1167.
Can I keep driving? Safe to drive. Without the heater the sensor warms up slowly, so the engine runs longer in open loop after each cold start, costing some fuel and raising emissions until it's fixed.
How to read P1166
- P Powertrain
- 1 Manufacturer-specific
- 1 Fuel & air metering
- 66 Specific fault
Symptoms
- Check engine light on, often shortly after a cold start
- P1167 stored at the same time
- Worse fuel economy on short trips
- A/F sensor takes a long time to become active in live data
- Car otherwise drives normally
Possible causes
Ranked by how often each cause is behind this code (editorial estimate).
- Heater element inside the A/F sensor burned open
- Blown A/F sensor heater fuse, often from a shorted heater
- A/F sensor heater relay not switching or with burned contacts
- Heater wire chafed, melted or broken near the exhaust
- Corroded heater pins in the sensor connector
- PCM heater driver fault
Diagnosis step by step
Check the heater fuse
Find the fuse labeled for the A/F or LAF sensor heater in the under-hood box and check it with a meter, not just by eye. If it is blown, look for a shorted heater or pinched wire before fitting a new one, or it will blow again.
Tools: Multimeter, Flashlight
Test the heater relay
On models with an A/F heater relay, swap it with an identical relay from the same box, such as one for the horn, and see if the code clears. With the engine running you should hear or feel the relay engage shortly after start-up.
Tools: Screwdriver
Measure the heater element
Unplug the sensor and measure across the two heater pins on the sensor side. A wideband heater is low resistance, typically only a few ohms at room temperature; compare with the spec for your engine. An open reading confirms a failed heater.
Tools: Multimeter
Check power and control at the harness
With the relay energized, the heater feed pin on the harness side should show battery voltage. The other pin is switched to ground by the PCM; if power is present but the heater never gets a ground, check that wire back to the PCM for an open or a short.
Tools: Multimeter
Replace the sensor and clear the code
If the heater is open, replace the complete A/F sensor, since the heater isn't serviceable on its own. Clear the codes and let the engine go through a cold start and warm-up to confirm the heater monitor passes.
Tools: O2 sensor socket, OBD-II scanner
Where the parts are
- 1Battery
- 2Alternator
- 3Engine computer (ECM)
- 4CAN bus
- 5Transmission
- 6Wheel speed sensor
- 7High-voltage battery
Repair options & costs
| Repair | DIY? | Estimated cost |
|---|---|---|
| Replace A/F heater fuse (after finding the cause) | Yes | $1–$10 |
| Replace A/F sensor heater relay | Yes | $20–$80 |
| Replace A/F sensor yourself (OE-type part) | Yes | $90–$250 |
| Replace A/F sensor at a shop | No | $250–$550 |
| Repair heater wiring | No | $100–$280 |
Rough US estimates for parts and labor; shop rates vary by region. Not a quote.
Emissions / smog test impact
Will fail
P1166 keeps the check engine light on, which fails an OBD-based emissions test, and the O2 sensor heater monitor won't report complete until the heater works. Allow a few cold starts after the repair before retesting.
Frequently asked questions
Why do P1166 and P1167 usually come together?
Both watch the same heater. If the heater circuit is open, the PCM sees a wrong voltage on the control line (P1166) and also sees the heater not performing (P1167). Fix the electrical fault first, then see if P1167 returns.
Can a fuse really cause P1166?
Yes, the A/F heater usually has its own fuse. If the heater shorted internally, the fuse blows and both codes set; a new fuse alone won't last until the shorted sensor is replaced.
Is it just the relay? It's much cheaper.
It can be on models that use an A/F heater relay. Swapping in an identical relay from the same fuse box is a free test. If the code returns with a known-good relay, measure the heater element next.
Does P1166 affect the engine long term?
Not mechanically. It costs fuel on short trips, raises cold-start emissions, and may make the catalytic converter work harder, so it's worth fixing reasonably soon.
Sources
- Manufacturer public service information (cross-checked)
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