Emissions code
P0491: Secondary air system flow too low (bank 1)
How this range is built
| Part of the job | Low | High |
|---|---|---|
| Parts | $150 | $700 |
| Labor: 1 to 2.5 hours × $125 to $175 an hour | $125 | $438 |
| Fees: shop supplies, disposal, diagnostics | $0 | $25 |
| Total, rounded | $270 | $1,175 |
The hourly rate is the BLS Occupational Employment and Wage Statistics, May 2025 average pay for this trade in Boston, $31.54 an hour, times 4 to 5.5 to cover what a shop bills (overhead, tools, insurance, profit). Parts are a band from budget aftermarket to the manufacturer part. Full method.
Not a repair shop, estimates only. Typical range, estimate. Not a quote. Your car, parts and shop change the number.
This is the range for the most common fix, secondary air injection (smog) pump replacement. Which repair your car needs depends on a proper diagnosis.
The secondary air system blows fresh air into the exhaust on a cold start. For the cylinder 1 bank, the computer measured less air arriving than expected. Carbon and moisture can clog the small ports in the exhaust, or a weak pump, a stuck valve or a cracked hose can leave that bank short of air.
- Safe to drive?
- Usually yes, for now
- Most common cause
- Clogged air injection ports
Does not change driving. Fix for emissions.
What you may notice
- Check-engine light on
- No change in how the car drives
- Possible whine from the air pump on cold start
Common causes, most likely first
- Clogged air injection ports
- Failed pump
- Stuck or leaking valve
- Cracked hose
Check this first
Look over the air hoses and check valves for blockage or cracks, and listen for the pump at a cold start. Ask the shop whether the injection ports were checked for carbon, since a clogged port limits flow.
Other repairs that sometimes fix P0491
| Repair | Typical range, Boston |
|---|---|
| Diagnostic scan (check engine light) | $60 to $290 |
Before you pay
- Ask the shop what test showed this part is the cause, not just that the code appeared.
- Ask for parts and labor listed separately, and for the hours.
- Ask for a written OK before any extra work. Massachusetts rules require written authorization before repairs.
Got a quote already? Check it against the range. Want to see if your car has a recall tied to this problem? Check recalls by VIN.
Need to read a code yourself? OBD-II scanners on eBay (affiliate link, disclosure).
P0491 questions
Why clog?
Carbon and moisture build up inside the small passages where the air enters the exhaust. Condensation from short trips makes it worse, so cars used for short drives are more likely to clog.
Cost?
It can be high, because clearing the ports means removing exhaust parts and the pump can be costly. Ask for a diagnosis that shows which part failed before you agree to a replacement.
Pass inspection?
Not with the light on. The system does not change how the car drives, but it is part of the emissions equipment, so plan to fix it before your inspection date.
How much does it cost to fix P0491?
The most common fix, secondary air injection (smog) pump replacement, has a typical range of $270 to $1,175 in Boston. It is an estimate built from parts prices and local labor wages, not a quote. Your car and shop change it.
Related codes
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Code meaning written in our own words. Some codes mean different things on different makes, so check your scan tool or manual. Not a repair shop, estimates only.